When sealing round a bath or shower, make sure you are standing inside it, since the plastic deforms more than you would expect. Even better to do the bath with it half-full of water.
Please don't install a bath onto a covered floor - mount it to the floorboards and put the floor covering round it.
From experience, I would install all the suite and then tile around it, even if you are tiling the whole wall. It is what was done when we replaced the suite and we thought about tiling the wall first, then fitting the basin to it, but ended up fitting the basin to the wall and tiling round. If the basin needs replacing, we shall create a contrasting splashback area.
Plastering is a job best left to experts, but the cost of patch-plastering (small areas and repairs) will be excessive and if the patches are relatively small you will have an even surface to match to.
Hope this helps.