Strange couldn't really control all of that power, which is why he needed to bleed it off through dimensions as he returned to Earth. By "backed by Death" do you mean she just prevents him from dying or is directly intervening in the fight?
Yeah I mean Thanos can't be killed in the conventional sense because Death won't let him die. I figure thats the only way he'd be a match for all the heralds at once.
As for Strange not being able to control all that power thats not true, Kaluu only said as a consequence of him occupying a plane with such highly charged energy or whatever his power would unconsciously destroy galaxies if they just ported to earth's dimension.
Strange lost his ego and wasn't really Dr Strange or Shuma at that point he was just a being of power who could do anything.