Since I haven't seen a clear answer yet...

1) Why does [Role] in need not get me instant queues when I queue as [Role]?
Because the system doesn't update in real time. You and 300 other people saw it say Tank in need, so the first couple tanks might've gotten instant queues, but now there's more tanks in the queue than there are healers. I don't know how often the [Role] in need system updates, but it doesn't roll over as soon as the tanks surpass the healers.

2) Why can I still get Sastasha and other low-level duties when the entire party is clearly way over the level of the dungeon?
If all party members were queued for the roulette (rather than those queued for specific dungeons), then it's up to RNG which dungeon you get.

Obviously, I didn't build the system, so take these answers with a grain of salt. These are merely my own assumptions.