These questions are amazing.

A bit late to this but can confirm sleeping with her and then killing her does not affect other romances. In my Dark Urge playthrough I did this and then got with Lae’zel later.

This is an interesting question, in my 600 hours the only characters I’ve seen have romance starting dialogue outside of the Act 1 party is Lae’zel and Shadowheart. Lae’zel got an exclamation bubble above her head after I fought the owlbear, where she basically says what she says at the party but potentially sooner than the party (this didn’t trigger the sex scene till the night of the party even though she asked way before). And Shadowheart gets a dialogue that says something like “I feel we missed a chance to connect” but I’ve only seen that dialogue actually appear a handful of times, I imagine if you kill Minthara you won’t be in a relationship and thus the dialogue should trigger for you, though I might’ve actually been in the shadowlands actually when I got it, I don’t recall exactly. Wyll will try to seduce you with a dance in Act 2 which I believe starts his romance. But I’ve never been able to start a romance with Karlach, Gale, Lae’zel, & Astarion post Act 1 if I didn’t ask during the party.

I still regret breaking Mintharas neck…

I know not killing but sleeping with Minthara still allows you to romance others.

So I imagine killing her would just be added security in that case.