Yes, you'll need a pitot tube mount like the one linked below. I'm not familiar with the RANS but they likely have a forum like this for some assistance.
https://www.aircraftspruce.com/catalog/inpages/gretzbrackets.php
Infinity grips:
PTT
AP Disconnect
Radio FF (rarely used - wouldn't do it again)
Trim (coolie hat)
Flaps
Missile launcher (unused, really, but that's what I tell passengers)
Only put engine start if you have a separate start enable/disable switch
Ident would be seldom used, IMO. You are already...
I installed the older GPS-250 and the 2020 only because I already owned both. Down-side is that the 250 uses a serial input that I may need for something else down the road. When that happens, the 250 will go away and likely have an IFR navigator takes its place.
Resistors are only required for magnetos. Not need for electronic ignition (per the install manual). You mentioned PMag, which is a different EI product. It also does not use a resistor for RPM/Tach.
The Ethernet cable has nothing to do with synchronizing settings between screens - that is done over the SV network. The Ethernet cable allows the monthly map updates to flow from one screen to the other - so you don't have to update each screen individually.
I think you may have misunderstood what Mike means. You need to know where the cylinders peak, and they ideally peak about the same time, to know when you are ROP or LOP. That doesn't mean you have to go to peak each time you want to go ROP or LOP. I don't use the lean-assist function either but...
If using the Dynon transponder, it gets its data from Skyview. Calibration of the AD-AHRS, as I understand, is done by Dynon. If you have an issue with incorrect altitude, it is likely related to a static leak, which you'll need to find/fix.
When the HDX first came out, it was compatible with SV Touch/non-Touch but they said then they did not support a mixed install and updates to the systems would eventually remove compatibility.
They can both go to a common ground. If you are using the Dynon AP button panel, it provides relay functionality. If not using it, you will need trim relays. I wired a switch into my panel to disable the grounds to all but PTT so I can disable the buttons on my co-pilot grip (Infinity). More...
You're going to want EGT in the engine strip so you can properly lean during climb and cruise. Once you know where the cylinders peak, maintain 150-200° ROP during climb. Once %HP is below 75%, it is safe to operate LOP, so you'll want that widget too.