Increasing subfloor thickness under new shower pan
1/2" is a bit thin to span that distance, although it should not be a problem with a concrete slab above it.

If you are worried about it and need to use 1/2" sheathing for other reasons then just install 2x blocking perpendicular to the joist at a shorter distance on center, say 8"-12" O.C. in the area of concern.

As for the wall studs, we would typically leave the wall as is and shore it up from underneath with blocking, also installing blocking along edges of the new sheathing.  Therein the new sheathing would not run under the wall and the wall would be now supported by the blocking.