How to address diverter pipe length issue during shower remodel?
Adjusting the length of the tub spout pipe will be way easier than changing your entire shower enclosure and tile plan. 

The method you use when you shorten the pipe (and how much you shorten it) will depend on the type of tub spout you will install (different spouts have different pipe-attachment arrangements, not all of them thread on like your old one did) and where your finished wall surface ends up being. 

I highly recommend that you do not cut the spout pipe until you are done tiling, so you can ensure the proper pipe length.

P.S.- some spouts just slide on and use a gasket to seal; they are more forgiving when it comes to pipe length: