Lynx-o, Pumyra and Bengali were already Good Thunderians.
I know, it’s just a hypothesis laying emphasis mainly to the Good turned Evil warrior and losing the pros and cons of the Good side of the “force” as well.
Your theory of getting accustomed to the Thundrainium chunk of prison is a plausible explanation and a great idea.
Wizz-Ra was a powerful sorcecer, no doubt and he could imprison Grune but he was banished forever to the 7th dimension by the Ancient Sphynx after he lost a battle to Mumm-Ra. It is said that he reappears every 7000 years when the dimension is weak, which is a much longer timespan than Grune’s first appearance in Third Earth centuries ago, according to the two Bolkens, so i don’t think he had ever any meddling with Grune.
The Grune character, for me, was one of the most interesting ThunderCats ever showed. The whole ‘good guy turned bad’ is always a fantastic story and the episode was so well written - especially when, initially, the whole Cats Lair is shaking and they’re absolutely powerless to stop him once he shows himself - spirit form. No other bad guy really ever did that in the show - a favourite of mine. I still have the action figure - chokeslamming my action-figure Tygra from large heights was a favourite of mine as a kid.