Thanks for all the help, everyone.

What does a "supported upgrade" mean? On CO, I would obviously only be able to upgrade one companion, so in this scenario (in the best of circumstances), each plat elite could bring one companion, so a maximum of 4 in our party of 5 could upgrade.

I have read somewhere before in this forum that somebody got multiple advance upgrades (1 elite, 2 companions) in a single UA PNR. I don't know if that was a fluke, though.

So it seems like I want to split up this PNR into either 2 or 3 reservations.

And what about substituting my new Premier Exec number for my CO Plat number? Will that make any difference at all in priority?

As far as Economy+ goes, UA does seem to enforce the one companion rule (which is different from CO's elite seating), so we're only able to hold 4 Economy+ seats. What we typically do in this situation (party of 5) is hold 2 window/aisle pairs in Economy+, book the best aisle seat we can get in regular Economy, and hope one of our middle seats stays free.