how much water do you take out in each water change? if you only do about 25% at a time, consider a more drastic amount for the next wc, as soon as possible...50-100% would be good, just this once.
the heat will definitely make him happier... so good move putting the light over him. just make sure the temp doesn't get too high.
your tank is too small to go that long between water changes. i have a betta in a 2.5 with a filter and his water gets changed about twice a week. so changing Maxx's water more often should help keep him happy.. every three days or so should be good.
the clamped fins are most likely a sign of yucky water conditions... goldfish are poop machines (as it seems you have noticed... lol), so i can definitely tell you that Maxx has been living in much more ammonia than he would have wanted while the two of them were in there together. that's why i suggest a big water change... to make sure there is no more goldie poop in Maxx's home.
aside from a lot of ammonia, or nitrite in the tank, you have to watch out for fungal and bacterial infections. betta fins are for some reason exceptionally prone to both. if it looks like something is growing on them or like they are slowly melting away, you need to treat him with medications.
This filter would be good for your little tank.
oh and i wouldn't worry about him spitting out food... i see my bettas spit and re-eat their betta bites five times sometimes before they decide they're going to stay in their stomachs.
hope this helps.