Jeff said MMR is expressed as something like “+3” or “-2”. He didn’t go into much more detail on that, but he essentially said that they don’t display the MMR to the player because the player would not understand how to interpret it.
MMR will display recent trends where as SR displays current state and is easier to understand.
The problem with a “stress test” match is that it often requires drastic changes to someone’s normal play style. I play a lot of Rein and do great when I can get at least one healer to pay attention to me and at least one DPS to help when I’m trying to hold a corner with the enemy creeping in. Getting an off tank with some synergy is basically game over. During what I can only assume are these “stress test” matches I play the same way as normal games but consistently die trying to hold easy chokes while literally 5 teammates struggle and fail to kill a tracer/doom/genji in the back lines. The game then devolves into trying to swap to handle whatever the other teammates can’t or won’t deal with and babysitting rather than playing your preferred style. These types of games aren’t fun for anyone on the team and certainly aren’t testing “skill” in a heavily team-dependent game.