Just an idea, but would it be a good Comeback Mechanic if units that passed through your gate gave you crystal? Whilst this wouldn't allow an immediate comeback, one of the larger setbacks is investing gold into collectors for the second half of the population game.
If one was practically handed defensive mechanisms to either A) invest into the population game, B) invest into mercenaries, or C) invest into heroes to farm units on the other side, would that be a good set of tools to give a player to make a comeback?

Above this, I think the Sell-For-Full-Price ability should perhaps be triggered by something, i.e. not just a once-off for a losing player. Perhaps every 50 units that pass through your gate should trigger it? Or, if you go with the idea above of giving a losing player more crystal, then you could tie the Sell-For-Full-Price ability to a crystal cost. This way: if a player has an abudance of crystal whilst they're losing, they can invest that crystal into changing their compositions rather than other comeback mechanics.

I really think the key to a comeback mechanic is giving a losing player a set of options that aren't all necessarily good for the situation. If you give them crystal for losing and change the Full Returns Sell function to be tied to crystal, then you've got a lot of options that don't involve overpowering your opponent in a brainless spamming of units. It keeps this game's fundaments intact.

First of all sorry for my english :)

I fully agree that its quite impossible to make a comeback when you have bad start and you dont have counter for enemy units even after 2nd swap. So I have some offers.

1. You can change bounty for units that walk through from what you have now to 60% for example. Its not bad because you're not only denying gold for killing your army for your opponent when it walk through, you also recieve a lot of gold for yourself.

2. When you have more than 5k value you start to recieve only 50% bounty for your units, when you have 8k you recieve 45%, when you have 10k only 40% etc. and when you have 15k and more you recieve only 10%, so your enemy still can defend base.

3. Same thing with your legion's skill (Wraith King ultimate) you get not full price for units passing by.

4. Building towers is not good idea, it will make game tiresome.

Also I have a lot of experience of playing Castle Fight and Legion TD, its kind of similar maps in Warcraft III TFT. And I would be happy to assist in creating this map if needed.

just dissable gold gain by passing through the portal what ever this is .. problem solved .. would be way much better this way .. maybe get crystals insteand .. would be way much more better ...