bath tiling

[wasim21k]

Hi all,
its time about time to do some tiling second bathroom. i have few question where i m not sure what to do so i thought asking here fetch me some help.
i have removed all the old tiles, and paint is striped as well from all the walls and i m going to tile full walls:

i have removed bath tub to see the damage on the wall and in corner where bath tub sits beside wall, it all looks ok but due to water seeping through the tiles there was some damage to plaster board but does not look very bad (see picture).
now million dollar question: do you think tanking would be enough ? or do i have to put some backerboard and tanking on backerboard?

bath overflow & waste: why bath waste made of plastic and bath sink from mettle ? i had to fix that waste/overflow which isnt easy to access and amount of trouble it caused me in past is just unreal so may be i could get some decent waste/overflow this time to avoid this problem altogether? any suggestion on fitting waste/overflow?

the mixer tap: these things are even harder to replace so any idea while i have everything open? i.e put some flexi pipes/move mixer on the wall from bath tub itself?

Side panel: what would be best to use as side panel on bath tub? i m not going to tiles that as it does not looks nice.

trim/PVC between wall and bath tub: should i use caulked or silicon with trim? or any other and better options?

ps sorry for all these questions, i wanted to get this job done right.