Grout lines on the shower wall border tile is 1/8" -- I'm planning to use unsanded grout for that. However the gap between the bottom tile row and top of shower is 1/4" strong. Must sanded grout be used for that or can I get away with using unsanded grout for that also?

Hi Good Evening, finally I found a good place to get my answers I initiated to remodel my bathroom (started with powder room). Powder room sub-floor (3/4 inch) is plywood and Underlayment(1/2 inch) was also ply wood. While removing tiles and adhesive, undelament got damaged. So, I decided to replace undelayment and successfully removed it. Now, I am in a confused state on which approach to follow for undlayment, Approach 1: Go with 1/2 inch plywood Approach 2: 1/4 in plywood and on top 1/4 backer board. I seriously want to go with baker-board approch, but since thin-set is suggested to use, I don't want to use bakerboard(1/2 inch) directly on sub-foor, since it will hard to replace again(if required to replace tiles) and I don't want to damage sub-floor. Please provide ur suggestions for my clarifications: [list] [*]Does my approach 2 make sense and good to go. [*]What type of thin-set to be used for bakerboard? [*]For Approach 1, can I directly apply thin-set on plywood for tiles or do I need to use any adhesive before applying thin-set? [/list]