So after inserting the spray foam up through the ceiling in the kitchen, into the tub crevice...
it seems we finally have a winning solution.
As a reminder, this is the link to the *before* video.
I AM SO HAPPY.
I think I'll take a shower.
Or a bubble bath.
Addendum: If you're building your own tub surround, installing a new tub or whatever... don't wait until the drywall is up to make sure your tub isn't a squeaky annoying project waiting to eat up another 3 days of your life. Get in the tub and walk around. Make sure it's seated well and if it's not, get the spray foam.
Or better yet, follow the directions on the tub insert and install the damn thing in concrete, avoiding all of this mess in the first place.
Ah the 20/20 hindsight.