Not sure what might be considered easy but, I would cut a 1x1 to length and put 1 inch screws similar to decking screws on each end and partially screw it to the ceiling (leave enough space to slide some tiles under it). Then I'd butter the back of the tiles, slide them into position and tighten the screws to hold them in place until the mortar dried. Then unscrew the 1x1 from the ceiling.

Grout is important.When grout does its job  locking tiles tight, keeping out water, and giving floors and walls a finished look  nobody pays much attention. It's only when grout fails, becoming stained, cracked, or falling out altogether, that people take notice. But grout deserves more respect. "Not only does grout fill the voids, it makes the floor, wall, or countertop stronger by bonding the tiles together and preventing the edges of a tile from chipping and cracking. Cicliot

First you need to remove the tile and prepare the floor for install you do not put this new flooring on top of tile,I also would stongly suggest yo not try to do a herringbone pattern at all.

My eyes are closed and am holding my breath for you... NO... there is no possible way at this time for this to be fixed. If you hired a professional I would demand them to re-do it ALL... First off, the tub appears no not be level which if this tub was existing the tile setter should have installed a proper tiling flange at the bottom to prevent this from happening. Why are your walls "built up"??? It looks like they added another 1/2" to all sides for the tile... but why!!! Why is the shower arm still installed and now back in the wall? no, no, no, no, no!!!!! Did you select new fixtures or are you re-using the existing?