Using RUC but...
 

As a DM, whenever I book travel with companions (with the same PNR as me), they get access to C+. If I decide to burn a RUC on a trip, will that somehow cause my companions to lose their C+ seats? On the one hand, I can't imagine why it would: after all, if my RUC doesn't clear, then I'm still with my companions in C+. But, if I clear in advance, does that jeopardize their seat assignments and SP access?

I really don't think DL does any kind of checking up on this kind of thing. I am GM and my spouse is plat. We have booked together on the same PNR and then he last later switched flights and I was fine keeping my seat at no charge.

Generally, if your companion already has the seat, it will stick. If you clear the upgrade in advance, and they later get moved out of the seat by an equipment swap/schedule change, or even if they want to change to a different EC seat, you will be charged.

Using RUC but...
 

Here's another data point: right now, I'm waitlisted for the UG -- and so are my other three companions. I called the DM desk and she noticed that they're also in the queue, even though they aren't. The issue seems to be that all of us are on the same PNR. DM agent said that splitting PNRs made most sense. She did say, however, that everything in terms of coding the RUC is correct and that it's for one person (me). We both wondered whether this UG would go to the gate (BF upgrade)and then the GA handles it there -- though I worry that will result in split PNRs, especially on the return flight.

All passengers on the same reservation must be in the same cabin and class of service, so expect to split your PNR at some point. What I don't know is whether you can successfully clear (in advance or at the gate) if you have 3 companions on the same PNR.

Sort of. If nothing changes, they'll keep their seats. If there's a substitution or irrops, anything can happen. A polite request to the diamond desk might make it unhappen.