Tirek probably went into seclusion, maybe praying on a stray pony or two who came his way, until he could really function. I don't see how the timeskip for him from IAT to TK is a blatant plot hole.
I also thought Discord was done surprisingly well, despite me bringing up my general complaint about him again this morning. Tirek tempts him with basically free chaos, but
Discord still wants to be friends with Tirek, and that's how Discord is so easily betrayed. Discord even reminiscences a thing about Rarity at some point too. The message of friendship got through to him [weakly], and now that he's gone through this trial, he's come out much farther along the path. Just not even worth a throne yet.

But yeah, secondarily this was a Discord 2-parter that helped his character a good chunk while also serving the plot as the last inspired creature for the 6th key. Pretty solid writing.
I am pretty sad Golden Oaks had to go for Castle Playset Deluxe. I can only hope Castle Playset Deluxe becomes as roomy as Golden Oaks, especially since the Mane 6 are set to hang out there a lot, I think.