Yes, maybe the teaspoon per gallon WAS too much, not all bettas can tolerate it. Twitching is definitely a sign he was NOT liking the salt. If you want, you can try the tablespoon per 5 gallons for 5 days and see if that helps him.
Well, it sounds like another reason Stampy's fins are a little torn is because of the water quality. For a 5 gallon tank that has a filter, you'll want to do one 50% change a week. If the tank doesn't have a filter, it's two changes a week, one 50% and one 100% (it doesn't have to be 100% per se, it can be 95% if you want to leave Stampy in the tank). You can also do two 50% a week. Unless Stampy is sick, like he is now, the only thing you'll ever need to add is the Stress Coat or Prime.
If you get on a schedule of weekly water changes, then I think Stampy's fins will begin to heal up quickly.
The poor water quality means there's a lot of ammonia in the water that can burn his fins and gills, that's probably why his fins are shredding.
Bettas are really quite trainable, aren't they? Although you're right, Stampy probably was thinking food when he went for your face, it's still cute that he could be trained to do that. However, it's more incredible how easily our bettas can train US. They wiggle and give us the "look" and we just melt and want to feed them.
It's hard to resist but it's for their own good that we don't feed them whenever they beg, haha.