1. By "VR Viewer" do you mean a VR headset? It you do, you have to understand that a headset is just like a new screen for your computer. So the performance depends more on the computer behind the headset.
That said, desktop computers are usually more capable, graphic wise, than an iphone, so you may have better performance on them, and may not have to "reduce" your models.
Note that viewing a scene in VR will be slower than the same scene in non VR, as we are rendering the scene twice.
2.Not sure what you mean by "1face vs 2 faces", but as a rule of thumbs, having as few separate geometries as possible is more important than having few faces. Drawing 1 geometry with 1000 faces is a lot faster than drawing 100 geometries with 10 faces (note that sketchfab does some optimisations on your models in this regard).
I can't give you a magic figure of number of geometries and number of faces that will ensure good performance on any devices (so many devices, so many differences) but keep the geometry number as low as possible, and then keep the poly count as low as possible (in that order of priority).
3.File format has little to do with rendering performance.So it won't make any difference if you use obj or fbx.
Though if you experience crashes, could you be more specific on the process that lead to the crash (url of the scene, phone brand/os etc..)?
EDIT: you can also report the issue from your phone, filling this form. https://help.sketchfab.com/hc/en-us/requests/new
It will gather technical information we need (browser version, os, etc..)