Drywall – What to use to seal around fixtures when tanking a drywall bathroom

I am proceeding on the assumption that these aren't visible areas, so looks do not matter.

I think you were right the first time, I'd use expanding foam for those jobs. I've used it in all sorts of cases like yours and it has done well. It isn't going to crush anything, even flexible dryer hose.

The alternative would be to cut pieces of drywall with the right diameter holes and to screw and/or glue them into place around this material. You'd need to move those wires around some if you have enough slack. Once the drywall is in place you'd just use caulk or silicone on the remaining small gaps.

I'd use picture wire between the two D-rings to hang the picture. Leave some slack in the wire, as that will give you some scope for adjustment to make sure it can hang level.

Usually, I'd suggest picture hooks, but with insulation in front of the plasterboard, my suggestion would be to drive decent length screws into the studs, leaving the head protruding a little, and hang the picture wire over the screw heads