I'm not sure if Iv understood your post ?
Sorry poor post by me.
It is the limited guarantee you are suggesting giving that I am cautioning about.
If you tile something in a way that is not to accepted
standards then something goes wrong; it will be the tilers (the professional who should know better) responsibility even if they were doing it in away as instructed by the (non qualified) customer. That is my understanding from reading other posts on this forum.
So my advise would be (assuming no extra boarding or other corrections are going to be made, after pointing out the problem) is to tile the walls correctly with the correct adhesive bed etc so that you can guarantee the tiling part. The tiles to the bath can then presumably be sealed as normal along the long side and tap end, leaving only the gap at the other end to sort out. If this is within the limits of a flexible sealant to seal (with backing support) this is then the correct solution (even if a wide silicone joint is not good aesthetically) as this you can guarantee.
Personally I would silicone a white (Assuming that's the bath colour) tile edge trim (pointing upwards) or a uPVC quadrant over the gap as it is at the "dry" end of the bath.