A few questions about shims and mortar for tub install - with pics

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British Columbia
I'm finally at the stage where I will be installing the tub. It fits well in the alcove and is almost level. I think I will be able to get it completely level by adjusting the legs. There is about 2 3/4" between the floor and the bottom of the tub.
Edit: Since 2 3/4 is more than the recommended depth of the mortar, I will add an additional layer or two of plywood under that section of the tub to reduce depth of the bed that is required.

A few questions:
1) The mortar/grout
I'm planning to set it in a bed of mortar. I will be using this non-shrink product: https://basalite.ca/product/basalite-non-shrink-grout/

I'm in Canada in a small town so don't have all the normal products you may see in an American city. This does look like it will fit the bill though. Any reason to avoid this product?

2) Plastic
I have read in some places that I should place plastic on top of the plywood to prevent the plywood from sucking all the moisture out of the mortar (although I could paint on a sealant if that is the issue). I have read in other places that I should place plastic on top of the mortar to make it easier to remove the tub in the future. I do like the idea of using plastic on both sides in case I screw up and need to do it again or if the tub gets damaged in the coming years and I find myself redoing this job. However, I'm a little worried that if I have plastic on top and below that the mortar bed could slide around. I'm also worried about potentially hearing a crinkling sound when I stand in the tub. Perhaps those are silly worries. What is the best practice? One side, both sides, neither side? I assume standard 6 mil poly would be a good choice if plastic is advised?

2) Shims on the apron
Unfortunately, the floor is a little bowed with a low spot in the middle of the front of the tub. The apron touches on either side and floats in the middle. I imagine this will be less important if it is set well in a bed of mortar/grout but I'd still like to do something about it. I could level the floor with a patching compound but that's a lot of extra work at this stage and those may be prone to crumbing anyway. Is the best practice to just jam some shims in the low spot?


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