Chevy and Harmond made up once Harmond got kicked off Community.
But OP didn’t give the whole story about the season 3 wrap party and Chase refusing to do a scene. Harmond is known, anda self-proclaimed, perfectionist so the filming community would go on for hours past scheduled times frequently. Chase wasn’t a fan of working 12-hour days and working scheduled times. So Chase filmed his scenes for the season 3 final during his scheduled time and left. Harmond couldn’t get Chase back in time to film what he wanted to, so they had to change it. This led to a pissed Harmond getting/trying to get (I’ve read contradicting statements on whether or not Harmond succeded) the 100+ cast and crew to chant “fuck you Chevy” at the wrap party. Chevy’s family was at the party with him, so yeah, not the classiest moment from a drunk Harmond. So Chase called and left angry voicemails. Harmond decided to play these to the public.
What allegedly got Chase fired was him saying the N-word during a season 4 episode (Harmond was gone at this point). Allegedly, Chase was supposed to make a racially charged joke. Chase wasn’t a fan of how they kept making Pierce more and more racist, so this joke was the straw that broke the camel's back. To try to make it clear how angry he was and the path he believed Pierce was going on he said something along the lines of “just have pierce make a joke about n-words (he said the word).” This was also NBC’s and everyone else’s final straw with Chase and they fired him.

Now was Chase a victim in all of this, hell no. Chase had issues with everyone, but Alison Brie (they pretty much had an Annie-Pierce relationship). McHale wrote in his book that Chase would get physical with “joke punches” and that Chase resented him becauze McHale was a younger version of him (a good-looking, tall, B-list comedian). He was also jealous of Glover’s constant praise and everyone focusing on him. Chase allegedly didn’t want to be by Glover because Glover would take the spotlight (as time went on you’d notice more and more shots of just Chase, this was partially also because refused to put in extra hours). Then there was the fact he would constantly make inappropriate jokes.