I think Google cares more about the user experience (how bad is it if a user ends up on this page after searching) than the exact letter of the law. The truly sleazy sites usually do badly on both metrics though.
I've run sites with millions of urls spread across a lot of sitemaps, and Google will devour them without any fundamental problems. I found it wasn't uncommon for Google to regularly screwup the indexing of a few sitemap files out of a hundred or whatever, and often it would randomly fail to update a % of sitemaps in a timely fashion.
I will forward this on for SE to think about improving.
The question may not be answerable, but it doesn't make the question wrong. The question is nothing more than an expression of the users query or intent.
For example, unanswerable questions could give a stack listing if relevant answerable questions.
But it just seems wrong to take highly targeted inbound traffic and send them to a page which basically says 'nothing to see here'.
Or, quite often, substantial positive votes, and the wails of a legion of users protesting the closing. I would guess that that kind of question is more likely to be the target of enough inbound links to lift it into the search results people actually see.
Marking closed, zero- or negative-voted questions as noindex might be a good move, though.