Waterproofing Shower with BathFitter Tub

I’m re-tiling my shower and due to some mold/water damage or just bad material we ended up almost stripping everything down to the studs. Of the 3 shower walls we had to put up new backerboard on two of them.

Because of budget/time/effort reasons we cannot remove the original bathtub, which remains in place with a Bathfitter acrylic insert.

Everything I have read about waterproofing says that ideally the backerboard is installed up to the edge of the tub flange and waterproofing membrane (we’ll be using Redguard) should be applied over that joint to create a continuous waterproof surface.

Due to the way the original tub was installed, this is not possible. What’s my best option for moving forward so I dont end up with a leaky mess.