I'm a little puzzled that they even programmed in the option for you to be kicked out of the party though

If that series of events hadn't been quite so buggy when I played it, it probably would have been funny enough for me not to mind such an ignominious end to my poor character's journey

I disagree with letting self-imprisoned Tav show up - they're dangerous and starved. The fact that there's a saving throw check at the party is proof of that.
I agree it would be dangerous, but for me that's not a reason not to offer the player the temptation. And if it did go horribly wrong, I think it would be extra pathetic, with the character cursing themselves for their weakness when they initially knew better.
IThe spirit form attendance is already possible
Thanks for the link! It was seeing mention of a bug with attending the party in spirit form in the patch 7 notes that had led me to retry my ending. It's good to know where that option fits in, but yes disappointing that it doesn't apply to a character who chooses not to live as a mindflayer.
I despise the fact that ... we can't convince Orpheus and the Emperor to agree to temporarily work together because the situation is too dire for petty squabbles.
You make good points about the niggles with the way that whole scene plays out. But carrying on the theme of small changes that would make the ending less frustrating for me, I just wish that the Emperor could be persuaded to at least talk to Orpheus, even if Orpheus then subsequently refused to work with the Emperor (who has, after all, been stealing his power) and we had to pick between them anyway. In that scenario, it would feel more tempting to side with the Emperor, whereas currently it feels so wrong that he won't even try talking to Orpheus that I'll struggle to pick that option with most characters.
About Orpheus I normally don't feel responsible for him.
I freely admit I'm a sucker, but his more-honourable-than-thou emotional blackmail totally works on me, and it felt as though he could do so much more good than my characters had potential for. (Especially in my first playthrough where Lae'zel hadn't turned against Vlaakith.) But I do accept that not all good characters need to be so selfless that they'd usurp others' right to self-sacrifice. It just felt wrong in the moment for either of my two characters to let Orpheus do the honours.