If I read this right, you haven't done anything yet to check the drain beyond the washer hookup.Originally Posted by Sillycatnip
If I read this right, you cleared up the piece of drain up to the sink as shown in your diagram. That history shows that drain problems (plural) existed already, which adds weight to the hypothesis that there are now problem(s) downstream of what is shown in your diagram.
An unobstructed vent is also critical.
Washing machines spew out tons more water than any bathroom sink will ever have to drain.
FWIW, front loading washing machines use far less water; that doesn't solve your drain problem.
That's as much as I can comment. Others will help you more.