Can anyone give me the rundown on whether this is possible anymore? It seems like Aimpoint has discontinued the adapter plate for the P320 series. Is it just that popular and coming back on to the market, or is there something else going on? Also, I would like some recommendations on other RMRs for the P320.

Mounted a Vortex Viper on mine with a Springer Precision adapter mount.
Have not sighted it in yet.

I'd take a look at the Sage Dynamics video on the ACRO first. Battery life is very short compared to other optics and I'm not sure it's worth the money unless you've got a lot of battery money.

The X Compact, depending on when it was made, is cut for either the Romeo 1 (and I think a couple Vortex optics, but I'm not positive) or the Deltapoint Pro and Romeo 1 Pro. There are various adapters out there to adapt either pattern to fit the Trijicon RMR footprint (and by extension the Holosun 407/507/508. I would personally look at any of those optics.

If you want the best possible optic without an adapter, the Deltapoint Pro on the "R2" X Compact is probably the way to go. The Romeo 1 Pro may also be solid, but I've not handled one.

X-Compact is cut for Romeo 1 Pro and DPP, just like the M17. There was not an X-Compact with the original Romeo 1 cut. X-Carry yes, but not X-Compact.

Aimpoint mis-designed their first P320 ACRO mount. Apex has a mount now, without backup sites, or with backups sights in front of the Acro.

That appears correct. From what I have found, there is an "R2" variant of the X Compact only to keep the SKU consistent with the other guns. In other words, there is no difference between an original X Compact and an R2 gun (like there is on a full-size).

Makes... sense...I guess.