OK, some thoughts.
I think the writers had noticed that Grogar was showing up in fanart and were trying to use him somehow, given this would be the last-ever season of FiM. I give them props for trying to subvert the now tired "threat from a thousand years ago" plotline, but subtract points for how badly they wrote it.
Discord doing this is absolutely in character. As has been pointed out already, Tirek (and probably the others) would have no reason to trust and work for him unless he was disguised, and what better disguise than a legendary bad guy? He loses SERIOUS points for being instrumental in the wounding of the Tree, and am happy that the Mane 6 stuffed Sombra back where he came from after that one.
I am unsure about the Bell. Clearly things went badly wrong for Discord over that, but I think Discord's chaotic nature requires he build in multiple opportunities for failure in his plans (see also Xaos/Pandora from the El Goonish universe for this). The whole "sucking magic" thing is a plot hole here though; why they didn't use the Bell to remove the magic from those they captured (or just have Tirek do it) is still a point of confusion.
That crazy glue didn't "get a trial" before being consigned to Tartarus is a non-issue. She was put there personally by Celestia, who in a monarchy would be the final judge (and possibly in Equestria the only judge; they don't seem to have a court system)
If being turned to stone is worse than being consigned to Tartarus is a more complex question. Being confined for long periods of time without any opportunity for rehabilitation is a rather severe punishment, and I think less of Equestria for having it as an option. Being turned to stone, especially if you remain aware of what happens outside you, is even worse. If you aren't aware though, it could actually be a less abusive option - because it leaves the opportunity that (like with Discord) you could be unstoned later, rehabilitation attempted, then if it fails, stuck back in storage again until the next attempt.
I am also concerned about the wild (magical) animals trapped in Tartarus; they really shouldn't be there, and I suspect they were just convenient asset re-use rather than having a genuine reason for being incarcerated.