Full disclosure: I do not own a Pimax 8k X, but can chime in purely on the VR aspects of your questions. Also I was bored so here's the novel...
If your PC can run the SLR app or Deo VR through Steam and your headset can operate Steam, then the headset itself should have no issues whatsoever with it, whether it's a Pimax 8K X or an Oculus Rift (anything as long as it's pc-based). There's other methods for specific headsets (Quest 2, any Windows Mixed Reality headsets), but my understanding is that the Pimax can operate through Steam.
1.) As a general rule, always always always go for the higher resolution and larger size (more importantly higher bitrate) versions of the files, especially to take advantage of the resolution of the Pimax. Think of VR as just being a video projected onto a massive curved screen you're sitting in the middle of. The resolution of the file is the quality of the image being projected, and the dual 4k lenses in the Pimax just means you'll be able to take full advantage of the clarity of the larger resolution files. In general, it 100% most definitely makes a difference going above 4k (insert rant about misc. studios "upscaling" their videos, studios loosely using the term 6k/7k/8k, etc.). But tl;dr yes, bigger resolution and bitrate/file size is extremely important and worthwhile.
2.) I've tried downloading a few of the 120fps scenes available on SLR, and tbh I did not notice much of a difference at all on my Index. A bit smoother was about it. Then again, I could be wrong but I don't think there has really been any native 120fps releases from basically any studios, just interpolated ones.
3.) Two parts to this: the first is that there are many different slightly-different sources on this, but when you're looking straight ahead the human eye can see just shy of 180 degrees, it's only when you look out the corner of your eyes that your vision expands a bit past that 180. The other part of this is that when you need to focus or look at something in your immediate surroundings IRL, you almost certainly turn your head along with your eyes to do it. For example- if you're sitting down looking at your computer screen as you read this right now, you're probably not picking up the details of the wall or pictures in the background, the cup on your desk, that freckle on your mouse hand, or whatever else is in the periphery of your vision (though you probably are now that it was mentioned haha). I mean, just make a literal circle shape out of your fingers/hands and hold it a varying distances (we're talking cm's/inches) from your eyes: my Index is around 130 degrees max, the Quest 2 is a good deal less than that so both of those are a much smaller "circle", but the Pimax 8k X with 200 degree fov is basically like having no "finger circle" in front of your eyes at all, it's basically just your regular vision.
All this is to say: I genuinely don't know much about the 8K X other than what I've read on the various forums, which is that it's fantastic quality visuals but takes a good amount of work and tweaking to get working to it's full potential. Will also obviously require a high end video card, and will have to judge whether the price point is worth it to you personally, etc.