Welcome.. pretty boy you have.
He doesn't show signs of any fin rot- I would stop Maracyn immediately and either add in the carbon filter back into the tank, or do a 100% water change to remove the medication. Did you keep the tank dark while he was on it? Maracyn tends to be light sensitive. For future cases of fin rot I would suggest using AQ salt over medication, but make sure to get the appropriate dose and treatment period before using it.
A lot of bettas, if not all, will change colors once they get into a good home- either become brighter, or colors you have not seen in them will show up all of a sudden.
What looks to be happening with him if his fins are getting shorter is that he is biting them off- which is common in large finned bettas such as halfmoons.
The crimped could be from hard water- one of my boys who is older has crimped fins such as that and he is a fin biter as well. All you can really do is keep up on the weekly water changes to make sure fin rot doesn't develop and to promote fin growth. 30-50% weekly water change is what I suggest in that size of a tank, if the tank is filtered. Would also want to use a gravel vacuum/siphon to remove debris and waste each week when you do the water change. Don't forget water conditioner.
If there is no filter, then 1 50% weekly water change with 1 monthly 100% water change.
A diet high in protein will help his fins grow back and keep him healthy- small multiple meals a day is ideal, 2x + a day. Don't overfeed, their stomachs are approximately the size of their eye, so (in general) 2-3 pellets per meal twice a day is a good amount.