I'm so glad to hear he's doing well behavior and personality wise :D
if his fin and swimming are still funny, you were probably right about the injury. it may heal back up totally normal over time, or it may heal up deformed. but I suspect he will adjust to it and improve as time goes on... ^-^ since all his other behaviors are normal, I'd say it's unlikely it's life-threatening.. he'll figure out how to balance himself and compensate for whatever the injury may leave him lacking in. at least since he doesn't have a filter he won't have to battle that. ^-^
bettas sometimes injure their gill covers and can't open them for a couple days... but after awhile it gets better and they're able to open it again. so maybe the tail will turn out like that... at least he's swimming, that's a good sign.
Yeah, I've had similar experience I'm afraid... the sad thing is /basic/ bettafish care isn't that hard to learn fact-wise... so it wouldn't even be that hard to teach them and for them to remember. especially something as simple as water conditioner... I guess even a few minutes is more than they have to spare though. >_> and true to the bowl&vase thing, ha :/ some of them are gorgeous, sure, but those are plant bowls, not fish bowls..