Yeah, I'd be fine making an exception or separate option for "while hooked up to an external display". I think the user's intent could be reasonably construed there. Also if you're hooked up to an external display you often have external power.

Don't have a macbook, but on KDE that's already the default anyway it looks like--it's an opt-in to also sleep on lid close when there's an external display connected.

Why not crack the lid a little? No way I'm letting the display of my very expensive laptop contact the chassis while it's in operation.