Update 3:
Updated for Dharma compatibility
Unity loss from cultures affected by absolutism, the higher absolutism, the more decay from cultures
Unity loss from cultures affected by various government reforms
Civil war and independence war triggers reworked
Internal war disaster movement reworked
Faction leaders ascends thrones in civil war
Reworked most of civil war mechanics code to fix various problems
Factions now instantly appear at civil war end and should correctly get influence changes according to treatment of winning side
Starting resources in internal war for rebel side now based on faction territory development and condition
Ending event resets country stats, such as legitimacy, corruption, etc based on various factors, from faction support to
FOW removed if in supporting relationship
Call to arms and declare war bugs for internal war fixed
Can now declare war upon countries in independence wars (Not on rebel side/independence country)
In internal wars, vassals (except tributaries) and junior partners can now call overlord to arms if relationship is good.
The internal war will then continue with overlord as leader. Third party countries can still declare war on overlord, but not on the countries in internal war
Expanded unity from ruler
Fixed bug with non showing war icon
Fixed bug nogovernment
Staying below 5 unity can trigger a disaster which collapses the country and releases countries
Removed negotiation part of civil wars
Rebel sides ultimatums can now be negotiated through peace options in civil war
Various ultimatums added such as change religion, culture, ruler, capitulation
These options can be enforced or offered in internal wars. Overlords leading in a vassals internal war can also use these.
on white peace, both sides will gain autonomy
Civil wars: all peaces offers that doesnt end civil war will result in both countries existing and getting permanent claims over each other
list peace offers
New unity modifiers, forgot most but- Primitive state, and foreign ruler
Simple boom/bust mechanic implemented for talented/incompentent rulers (lasts till death of ruler)
Changes to wargoals in internal war, costs, goal, AE
Max warscore from wargoal changed from 25 to 40
Unity loss from estates now more gradual, scaling both with influence and level of disloyalty
Faction support reworked into various diplomatic actions. Establish a supporting relationship to perform these.
Supporting with troops or modernizing military will remove FOW for 24 months in target country
Added AI personalities, war exhaustion and war score as factors for peace treaties (ultimatums) during civil wars
Added AI personalities for choosing end treatments in civil war
Extended support logic and faction interaction logic for AI
Overlord joins subjects internal wars if good relationship
Changed country spawn mechanic if no core country free
Changed country color mechanic, now uses shades of mother country's color for civil war
Added bitterness loyalty modifier to old primary culture factions if rebels wins civil war
Faction resources has been simplified for performance reasons
ASsimilate nobility is possible now with either accepted culture or same culture group
Expanded AI logic for supporting internal wars and faction interactions/edicts
Added end treatment for independence wars
Internal wars can now correctly trigger when factions has above 90 influence no matter loyalty level
Fixed various bugs in relation to internal wars
Fixed bug with releasing factions always using faction 1's culture.
Fixed division by zero bug with a couple of faction variables
Reworked support system. A relationship is now established, and various interactions can then be performed directly from the diplomatic menu.
Various smaller tweaks, performance optimizations and bug fixes
bankruptcy morale and interest debuff reduced
Non-indepentt Colonial nations no longer has unity or factions
Balanced decay values from provinces based on various factions such as accepted culture, autonomy, culturegroup etc
Balance to various unity modifiers
Governments gets different amount of decay from culture - the more primitive the more decay

Update 2:
Cannot declare war upon a country in civil war or call to arms, and a country in civil war cannot declare war or call allies to arms
Internal wars is now represented through disasters
Starting process is now more refined
Internal wars ending expanded - various effect and choices on how to handle other side
Exhaustion after civil war gets added to devastated or persecuted provinces and increases loyalty of faction
Special mechanic for emperor of china - if low mandate civil wars will trigger as mandate wars. Among other things, if rebel side wins no tag switch, rather their dynasty will replace the old emperor
Mother country now always survives, (ie wont lose from accepting rebel ultimatum or being conquered by rebel side)
Fixed localization for wars, now shows correct countries in war name
Fixed bug with provincial debuffs staying after culture change
Made pending unity calculation events showing on load less jarring (Unfortunately cannot figure out how to eliminate it entirely - the event is already set to hidden)
Removed unrest from low mandate
Streamlined internal war mechanic-events/war declarations, negotiations etc generally should be alot more stable and less buggy
Fixed various CTDs caused by tag change
Fixed CTD caused by empty variable for religions / cultures during creation of internal wars
Defender of faith should no longer be called to arms if faction has other religion than mother country
Added more localisation for events
Fixed division by zero bugs and various other small bugs
Tweaked casus bellis, war goals and peace costs for internal wars
civil wars reduces unrest
New modifier awaiting new emperor (For low mandate china in civil war or mandate war)
A triggered modifier for hordes with a 6 military leader - Special boom modifier
If rebel side win civil war in Ming, country name will keep their dynasty
Rare rebel event triggered at below 5 unity now simply releases all possible cores
Various small bugs and balance changes to unity modifiers and faction monthly loyalty
Factions more likely to support civil wars
Fixed bug that caused various loyalty modifiers relating to ruler and consort culture and religion to not trigger
Chinese culture dont count in decay if emperor unless below 30 mandate
unrest reduction after internal war

UPDATE 1:
Faction influence now calculated without influence popups
Fixed various bugs with localisation in faction interactions
Added some missing localisation and added numbers for faction influence localisation
Fixed a couple of civil war/independence war bugs
Cannot support without manpower anymore
Can support primitives with modernize army, gives decreased tech cost, a general and army professionalism. Must be 9 levels behind supporter
Reduced years of nationalism from having faction present
Can no longer give factions territory to avoid confusion
Independence war is now defensive for breakout country (To avoid call to arms of host country's allies)
Various compatibility changes for ET, BT and Idea variations mod.
Civil war triggers faster when at low legitimacy

-FAction mechanic moved to estate screen
-Internal wars mechanic added
-Two diplo actions for supporting other peoples internal wars
-over 30 different events relating to internal wars, unity and factions. including a rare splintering event when at below 5 unity and unstable
-Added flavor various events that changes stability, loyalty of factions, unity, legitimacy etc, based on unity and faction loyalty. Including a rare event that can trigger at 0 unity, collapsing the country.
-Factions now has both a manpower pool and treasury which will form the foundation for creating a rebel army.
-Factions gain resources scaling with autonomy, but the greater the autonomy, the greater the faction expenses to maintaining the territory
-Decay from cultures added - unaccepted cultures costs a constant amount of unity. Lower this amount by accepting them.
-Added various faction interactions
-Interactions will affect resources gained from provinces with estate culture and loyalty
-Added enemy in war system to faction loyalty: faction loyalty drops if at war with enemy who shares religion/culture with faction and can military pose a threat to host country.
-Added faction loyalty modifier from looted provinces
-Being above 50 unity now grants boni
-Base decay is 0,2, down from 0,3
-Balance to loyalty loss from devastation (no longer includes provinces with raided coast)
-Localization updates/changes/fixes
-Fixed faction creation bugs
-Fixed bug with countries going below 5 provs still having factions
-Fixed broken disasters bug and a few other bugs
-Reduced war taxes unity effect
-added religious unity as negative unity modifier
-added overextension as negative unity modifier
-Modifier to unity for in civil war increased unity.
-Heartland no longer includes full dev value of same culture group at government rank 3
-Heartland is now calculated from primary culture dev and a smaller amount from same culture group
-Decay is now boosted by amount of unaccepted culture provinces relative to nation size.
-Balance to inflation, state maintenance and corruption from low unity
-Increased minimum autonomy in territories
-Epicenters and provincial trouble replaced by province debuffs from disloyal estates.
-Unity is now capped at 0
-Smaller countries less affected by decentralization
-Added buffs to unity for winning and loosing wars, lasting for 1-5 years depending on their magnitude and rivalry
-Factions who has accepted culture or same culture group now suffers less loyalty loss from unrest, devastation and looting
-Vassals and junior partners gets a small unity boost.
-Loyal factions give unity growth
-official language is now ruling class
-Estates decrease max unity
-Legitimacy affects unity only positively (from 0,7 to 0)
-celestial government factions loyalty loss from low mandate
-Edicts increase or decrease faction influence
-Debuff to loyalty from illegitimate heir or no heir during regency
-Faction Loyalty affected by victory/defeats buffs
-Faction Loyalty modifier during peace if neighbor is aggressive
-Various loyalty modifiers from faction interactions
-Alot of general balance, and probably alot of other things i forgot to mention

Supporting war effort loyalty buff (increased loyalty to factions and also reduced loss from devastation, looting etc):
-Enemy not same primary culture as faction
-No support if at war with country who shares faction rel/cul group if host doesn't
-Support on negative opinions or fearful of neighbor
-Rival war if faction shares cul group or is accepted
-Enemy is heathen
-Same side in civil war