I'd want to know the source of the ants. They generally need some water and some food, so if they are living behind the tiles, the problem is behind the tiles. Possibly, caulking would get rid of them. Possibly it would just hide the source of the problems. Going back to the old spout won't fix the problem.

The ants will just find another way in. Take care of the ants. Put out some Terro liquid ant bait.

I agree with Sigrid, there is most likely water and moisture and maybe even mold behind that tile. Was that tub/shower area, ever meant to have a shower? It is most likely not waterproofed for that underneath the tile, and water goes right through grout, if it was not meant for a shower.

You probably have water damage behind the wall, and the ants are feeding off the rotten wood.