Is my custom shower enclosure plan reasonable?

I’m renovating a cabin and my last inside project is to finish the shower/tub enclosure. I really don’t want to be doing this but can’t find reliable help locally. I have two big issues; the window in the center of the wall and I have never done tile and don’t feel this is the time to start.

I had tried to use a pre-formed tub enclosure but none of reasonable quality could be cut out to deal with the window. I had one that would work but it was so cheap and flimsy I decided not to use it. I stumbled upon a product called Duma Wall that I feel comfortable installing. Even though it is waterproof, it recommends a water proof layer underneath.

The tub lip is off the studs so to get the water-tight layers to hang over the lip of the tub I shimmed the whole enclosure out with 1/4" plywood. Unfortunately it has been very humid since I started this project and the plywood is warping a little. My plan was to cover this with 1/4" HardieBacker and then the Duma Wall. I have a plastic vapor barrier installed at the stud level. I plan to flash the window on the inside the same way I would flash it outside.

Is this a reasonable plan? Should I just tack the plywood down some more and assume the HardieBacker will smooth out any warping in the 1/4" plywood? Or just rip it out and do something else?

I’ve enclosed pictures of the tub and my plan.