Here's the thing, your trying to fix this backwards. The first thing I see is the bottom of the wall near where the tubs top is is all wrong and sticks out to far and will be like a funnel for water to get in.
Your trying to just find something to fill in that area after it's all done. All that tile and old concrete should have came off, So it could have been retiled starting at the lip around the surround and worked back to the inside corner and to the floor so all your cuts would be were there not seen as much.
That tub insert should have had tile board under the lip of it, all seams taped and covered with thin set, then red guard appled to waterproof it and all that old cement should have been chipped off so you would have a flat level surface to attach the new tile to.
Why are you using tile board above the surround? If the plan is to tile that area that's fine, if it's to be painted it should be paperless sheetrock.
This is one time that paying a real pro to come out and redo that whole thing will pay, not cost in the long run.