Attaching the moisture barrier to the tub's tiling flange is not at all necessary, but it's not harmful, either.
A full bead of sealant between the bottom of the wall tiles and the tub surface is, in my opinion, but that's the current industry standard.
I think it a very poor idea to intentionally trap moisture behind the tiles at the bottom of the installation. I have always grouted that joint, with full knowledge that the grout joint will fail and require replacement. At least the moisture that will
get behind the tiles will be able to escape at the bottom.
If you caulk, I would recommend you at least leave some gaps in the caulking for water to escape, but, again, that's not an industry standard procedure.
I deleted your other thread on the same question. One place at a time, please.