Well I don't know, why everyone is bashing that party so hard. Have you never been on any frat parties/summer camps/heard of happenings at Olympic village?
For f sake we've just erased half of the goblin population single handedly (oh yeah I enjoy getting in with Astarion in female drow disguise, then we steal their firewine and smokepowder and the whole place goes boom - glorious I tell you! Particularly that courtyard fight from the tower.)
Now we still face getting turned into slimy monsters at any time so this moment of celebration might as well be our last. If it was me I'd surely be getting drunk and well let's say I probably wouldn't say no to indecent proposal coming from the right direction.

Although I do agree there should be a bit of a warm-up before that comes up. Some party conversation and banter around campfire maybe? I think for these parties Witcher 3 is the best reference honestly, or maybe Mass Effect Citadel.

- I feel Astarion's party invitation is completely in character for him even if his opinion on character isn't the best, so I wouldn't change anything there.

- I've never actually gotten the Wyll scene - must be bugged or something, but in order for anything to happen the PC need to be the one to ask - so what's the problem? After seeing the cutscene on Youtube I have to state though that there's hardly anything more off-putting than a horned lady spying on you through eye prosthesis while you're getting busy.

- Gale will only act on it if you've been asking for it through the elusive and hard to get Weave scene. while I consider his "romance" development by far the best of all available options, the morning after talk though? IRL he'd be flying of my bed, clothes to follow through the window!

- Shadowheart is really sweet and conflicted about herself in general and her next day reaction actually makes sense in this context.

- Lae'Zel is uh just being herself I guess whatever that's supposed to be

In conclusion Astarion is the only one that doesn't leave the PC bruised the day after (emotionally or physically) which really wouldn't be my first guess... It really sucks to be left with these cliffhangers though for an unforseeable future.