It's okay Emily, you're not asking stupid questions and you're doing a very good job with him. It can be very hard knowing how to treat sick bettas because their symptoms can mean any number of diseases. So you're doing fine. And anyway, asking questions is the best way to learn. :)
I doubt he has fin rot. More likely, it's an issue of hard water and/or high pH. As long as he can swim, it's probably not a real problem. Curling/fusing rays in crowntails is pretty common, especially in hard water or high pH.
The excess slime coat CAN be a sign of an illness, such as external parasites but in this case, I think it just means the Melafix was doing its job. When a fish gets sick, its first response is to create more slime coat so it's harder for the bacteria/parasites to attach and infect. When that slime coat is no longer needed, it's shed and usually shows up as nasty white gunk in the tank.
I've heard crowntails can be more prone to constipation so we'll keep an eye on him. Not all fish poop every day though, so this could just be his normal . . . er, schedule. If he hasn't pooped in a few more days, we can try some epsom salt.
As for too much conditioner . . . generally, unless you dump half the bottle in, it's pretty hard to overdose on water conditioner. I almost never measure now, I just kinda of eyeball it. And it's always better to have more than less anyway.