question regarding backtracking (keys/etc.) mainquest unlocks certain keys...

[Matti]

so ive read that for example for a crystal screamer zombie in bel air that drops a key to curtis safe, you have to finish 10 mainquests or something in order for that zombie to spawn there.

my question to you is, i kindah wanna clear each map before i move on , but i cant right now cause the mainquest progress blocks me from doing so, and i dont wanna backtrack each time.
after how many mainquests (or the name of the quest) etc. can i safely go to the first map etc. to 100% clean the area and move on to the next one properly?

[TheSpearman]

A lot of later quests will have you going back to earlier areas, and you're free to move between the areas with the fast travel system later as well. So some backtracking will be unavoidable, but it's quick to do at least.

[Drake Ravenwolf]

You could just hyper focus on finishing the story line and then come back and (mostly) 100% an area in one pass - mostly because some unique zombie spawns are mutually exclusive of one another and/or of side quests in the area

[Matti]

weird design decision with all the backtracking then... i hate it...