I don't know where they've stated this literally can't happen.

Not that it can't, but shouldn't. With sources.

The problem is that parties instantly break all of these "rules" and completely negate their existence - not just bend them, break them.

Two solutions here: Find people to queue with or queue HL. No parties allowed there.

I also can't play the hero (or often even the role) I want in either draft mode, which is the whole reason for playing QM in the first place. If I wanted to be stuck playing tank/support forever in ranked, sure, I'd just do that (although not this close to the end of the season either).

Additionally, with voice comms coming it's effectively going to be parties in ranked too - those who enable and are willing to use the feature and those who don't/aren't.

Not allowing people to play as teams or restrict what characters they can play would be pretty hindering imo.

Then don't match solo queuers with parties of 3, period. Being matched with 3 memelords who run crap heroes and just solo push/feed/ignore objectives ruins the game for the 2 suckers who are stuck with them, and there's nothing the solo players can do about it. When it's one random, sure, you can usually manage in some way, but not when it's three people encouraging each other.