In the FAQ you have stated that units cannot retreat from co-existence battles. If this is true then it should be in the errata section not just FAQ. I could not find a rule in the game rules that state retreats cannot be performed by co-existent units. FAQs are usually to cover odd situations or circumstances where a rule could be played in a number of ways but this clearly should have been in the rules. Co-existence battles are generally going to be the first battles played out (due to corruption). If any users are aware of where the rules make it clear that retreats are not valid for coexistent battles then please direct me to the appropriate page.

I think it is correct that this is not in the Errata.

AFAIK the rules do not allow retreat from a coexistence battle, but this rule is just contained indirectly in the rules book and not eplicitly mentioned. It is probably because of the assignment of the designation attacker/defender in the coexistence battle.