alot of these questions im not sure myself, as im not sure what the original owners intention and motives where so im just going to make judgement calls for these questions.
1. though your right for the most part, its also true that some outsiders power are dependent of other things rather then followers. think ill give them the choice between being powered by followers or, that "something else". outsiders if they are powerful enough can create Boon's, for anyone really not just champions, though its probably makes more since that they would do so for them, given they would be more loyal. i'll have it so boons are manifested as a magic item that manifest a specific ability related to there aspect, allowing them to create up to 3, unless stated other wise.
2. to my understanding there isn't really a limit to how many outsiders there can be, nor are all of them in the main city or a city.
3. i think its just set that way to different between the good and evil side as the standard, though lets suppose there can be exceptions, i would imagine it would come with social consequences later down the line if not write away, because socially the idea of a good demon or a evil angel might be inconcivable though i imagine it would make for a intresting story line.
4. lets perhaps say the race is a angelic or demonic varient of there race when becoming a outsider, think of angels and demons not as a race but of a diffrence in moral alighnment with essence that grants various demonic or angelic traits.
5. while its said that outsiders despise other outsiders with the same aspect as the via " there can be only one" its not really explained how they deal with that... i'll likely make a system for that later...