If one could not OHP (due to low ceilings), would it be best to replace standing OHP with seated, or drop OHP as a core movement, and use movements like seated press, z press etc as assistance?
Seated press works fine.
If you can, try and do seated press on a backless bench. It will keep you from leaning too far back and turning it into a high incline press. Also will more closely mimic the core action of standing press.