No worries, happy to share whatever knowledge I can 
- Can you mix HMD and controllers from other devices plug-n-play?
Nope. The only possible exception to this is WMR controllers. Otherwise, you cant use Rift controllers with an Index, for example. You also cant pair more than 2 controllers per headset.
- What if you get 75Hz on both devices? What if on a 90Hz device you get 75Hz? IF 75Hz are good enough for one device, aren’t they good enough for the other device?
Aim for 90fps across the board. Chances are, anything above 75fps is going to look fine, but your target fps should be 90. In practice you may notice that lower framerates are acceptable, but this isn’t ideal. If your project can hit and maintain 90fps, you’re set. This connects to a broader discussion about performance and optimization, not to mention target hardware and requirements. One thing you can do if, say, your project is mostly hitting 90fps is perform an event tick calculation to find the frame rate and lower screenpercentage or pixeldensity when it dips under a threshold.
- What resolution is considered good enough to have clear visuals?
This depends on the headset. Older headsets with lower resolutions are going to have lower visual fidelity than newer, higher-end headsets by comparison. At a screen percentage of 100 and pixel density, you’ll see as many unique pixels as the headset is capable of. If this causes problems for performance, one option is to lower these values at the cost of fidelity and find a compromise between performance and resolution thats acceptable. Its all a balancing act
- No ray-tracing at all?
No ray-tracing at all, sorry. This is just straight-up not possible at this time from a technical perspective and even if it was, the cost on the system will render your game unplayable even on the highest performing machines.
- how are you going to achieve ‘realistic’ results in VR?
I’d recommend playing games like Half-Life:Alyx and Robo Recall for yourself. These games are about as good as ‘realistic’ visuals in VR gets. There’s a stickied thread in this forum that talks about maintaining frame rate in Robo Recall that includes info on complex models and high-end shading effects such as reflections. Reflection cubes/spheres work basically by drag-and-drop into the scene, and you can add them to your sky light to match ambient coloring.
- VR Frontend
By this I mean the program that windows runs to pipe data to the HMD. For Oculus HMDs, this is the Oculus app. For WMR HMDs, this is the Mixed Reality Portal. For Vive and Index HMDs, this is SteamVR. You can also run SteamVR for Oculus and WMR HMDs, but not without having the Oculus app or the Mixed Reality Portal running at the same time. For Unreal development (particularly with OpenXR in 4.27) this doesnt really matter. You could, potentially, turn a machine on and launch your project without a screen, but youd still need at least a keyboard to log into windows. The VR frontends will open automatically when the headset detects activity (usually a light sensor inside the face gasket).
- I can only assume you are suggesting 4.25 for commercial purposes
Correct. I wouldn’t even use 4.26 for hobby/side projects, since core functionality in the engine is straight-up broken for VR. Translucent materials, for example, fail to render in both eyes. I also had problems with certain dynamic elements not rendering properly. For utmost stability, stick with 4.25. 4.27, however, should be where your focus lies since the introduction of OpenXR and everything fubar’d in 4.26 is fixed.
- I see all these archviz VR demos on youtube… if ray tracing doesn’t in fact work, how can you achieve this kind of realism? Screen space reflections? Any tips/pointers are welcome.
They’re either using beast-tier machines for real-time lighting, a custom, source-code version of the engine to force certain functionality on in VR, or (much more likely) they’ve very carefully wrangled lighting builds for the highest possible quality static lights. Rectus_SA has expanded on this in a previous comment. Like I mentioned, its all a balancing act. If your game is performant in some areas, you may have the headroom to enable certain real-time effects.
I made a few comments in a thread some weeks ago that goes into a lot more detail about things you can do in the engine to juice performance: What are the Performance-killer for VR(eg.OculusQuest2)? all postprocess, all screen-space? - #2 by Bevman00
I hope that helps!