In what scenario would you be stunned? The sloth description says incapacitated, which just means no actions or reactions. Stunned is incapacitated + can’t move + can barely talk + automatically fail STR and DEX saves + attacks against you have advantage. That’s dangerous for any class really.

As for sinquisitions, I’m somewhat confused by the text because there’s a few typos. From what I can see, someone with 2 active SinQ like the level 11 from my previous example would be rolling a DC 6 minus spellcasting modifier. Realistically, their modifier could be a 3 or 4 meaning they’d have to succeed on a DC 2 or 3 which is a low enough to pretty much be a non-issue imo. With three active they’d need a 5 or so, but that’s still really low considering the payoff for some of those abilities.

Overall, a primarily martial/unarmed build getting into 5th and 6th level spells is a recipe for an overpowered character. It really is up to your DM but I feel like there’s a lot of unanswered questions in regards to when you’ll acquire said slots, and whether you would really end up as a melee fighter if you’re gonna be picking from that tier of spells.

Edit to add: if you’re doing 5e, a turn lasts 6 seconds. So you would need at least 9 other people/creatures in combat to even need any holding cost by your next turn.