yeah, DMs are the case that makes having a rendezvous relay proxying function make sense. at most the relay only sees traffic volumes and who connects to who, the content is encrypted. without much more elaborate than that you could use tor to set up accounts on several intermediary relays and voila, you don't even need tor to isolate who's talking to who, just two intermediaries and basta.

it can, but not if they are auth required, then you need a scheme to proxy out these auth requests. it can just be a simple http proxy too, though you'd have to control the bandwidth and perhaps limit it to what is listed in a user's relay list.