Stick to 4:3/5:4. Easier to handle since these are common resolutions, also has some advantages in regard to vorpX’s custom async timewarp. 1280×1024, 1600×1200, 1920×1440 depending on performance.
You should always try to reach 45fps. Depending on how sensitive you are to low framerates, you might consider 30 or so playable, but steady 45fps are required for really fluid gameplay.
The main difference between Geometry 3D and Z-Buffer 3D is that G3D uses two cameras like native VR apps (natural 3D effect, but slow), while Z3D only renders once and applies 3D in post (less natural 3D for nearby objects, but a lot faster).
The FOV in Skyrim SE is independent of the resolution’s aspect ratio, 120/120 is always the right value. For Rift it may be a bit lower. Try 110/110.