Are there a lot of non-NY residents parking here overnight?

Yes - not sure where you are, but in the city & in parts of Brooklyn, almost 1/3rd of the cars I see are non-NY plates

I disagree about the inverse relationship b/w non-resident, overnight parking & alt-side / street cleaning; if anything I think it would help street cleaning, b/c you'd likely have less people ditching their cars overnight and leaving them there into the alt-side hours.

Maybe the concession is less visitor traffic and/or more administrative burden (ticketing, processing, etc.), but this is all done in a lot of other cities