Hello,
The wait times in ranked arena are indeed regrettable. :( Unfortunately, players cannot be forced to play, and if there is no one to match, there are no matches. Rest assured, however, that if there are 50 players doing ranked arena, you will most likely be matching someone.

To clarify how the second question works, the gating procedure is a little complex, but makes sense. You can instantly match with someone of your rank on a win/loss streak. To match someone outside your rank, the game applies a timer, forcing higher ranked players to wait before matching lower ranked players. This protects the lower ranked players from being matched to the higher ranked players, and can create a situation in which the lower ranked players will match repeatedly without the higher ranked player matching any of them due to the forced wait time. Until one or more of the lower ranked players ranks up to a higher ranking, at which point they can freely match the higher ranked player.

As for the SSP rank, once players reach this ranking, they cannot match any other players unless they are SSP rank, again to protect the lower ranking players. The alternative of this is to see very powerful, SSP ranked players repeatedly farming lower rankings that want to match. We see this happening in theme arena, and we get a lot of complaints about it, so this is unlikely to change.