Starcraft2 has separate player ranks for T, Z, P, and Random. We should have that for roles.

No.

Then how do lane swaps work ;p

It's something that's crossed my mind on various occasions, and I'll sometimes refer to how for example I might be a diamond level support, I'm barely a silver jungler.

That said, while lane swaps are no big deal as the game can already determine correctly your role despite lane swaps (so the adc being an adc in top lane would still be 'bot' and the top laner playing in bot lane would still be 'top') and doesn't just go by your role at champ select, this does all presume that conventional roles are even being played. It's theoretically possible that there might be meta shifts or even just unconventional teams where there are two junglers or two duo lanes, or even a trio lane, and there'd be issues categorising that.

Additionally, how does matchmaking work even if we accept the conventional roles? My support mmr would be significantly higher than my jungling mmr for example, but theres no guarantee that I'll get either role (or if there is, queue times will be insane), or even a guarantee that I'd play the role I'm assigned. I could very well discover myself assigned jungle in a group for people who all have bronze-silver-ish mmrs in their respective roles, but plat+ in other roles and we'd rejuggle ourselves and utterly stomp our opponents.

Do we get mmr/lp for the roles we were assigned? Then it'd be basically meaningless. Do we get mmr/lp for the roles we played? Then gaming the system would be a huge issue, as people would intentionally queue for off-roles to get easy stomp matches, and rarely actually be challenged as they grinded LP for their main role.