Was the tub drain 1 1/2" or 2" . My area tub drains are 1 1/2" and showers are 2". Depending on how far the run is it may be building up back pressure because the vent is not close enough.
You clearly dont know any plumbers to make a statement they make as much as a surgeon. I on the other hand know a plumber, he is typing this message. He said he can guarantee he makes far less than any surgeon in this country. Maybe there is a mouse or hamster surgeon out there that may make...
I would consider something tied into the footer as permanent. I suppose the i beam route could be permanent, but it isnt my field so just having a guess. I would assume your climate location comes into account on what the permanent fix would be. Good luck
Get them stabilized with I beams. That will work until a permanent fix can be completed. You put 4 or 5 pieces of steel from floor to ceiling anchored at both spots tight against the wall. I have also seen horizontal supports between the verticals for extra holding power.