The number of turns not being in battle has nothing to do with healing received wounds. It's the not ending a turn in a settlement that makes the wound become worse.
For comparison, check the EDCT, trigger section for this starts with
and continues. So in order to have a chance to heal your wounds, your general needs to end his turn in a settlement, not outside of it and since he ended his turn in rebel territory and not in a settlement of yours, that happend.Code:Trigger wounded_serious_wounds_worsen WhenToTest CharacterTurnEnd Condition Trait Wounded = 3 and not EndedInSettlement and Trait Vitality < 6 Affects Wounded 1 Chance 60 Affects Wounded 1 Chance 30
Not a bug, works as intended.
Edit: as with regards to why he got the trait before you received the message, the trait is applied at CharacterTurnEnd and an autosave is generated until after all player turn end events (settlementturnend and characterturnend) have fired and before the next AI factions turn start events fire, while the message will only pop up at the start of your next turn start events.
As for receiving wounds for failed assassination attempts, no, assassination has no effect on the wounded trait.
As for best ways to heal a received wound: park your general in a settlement, leave all his movement points untouched (i.e. don't move him during the turn) and have a high tier healer in that settlement.