I have used the RCC when flying Dragonair when it is closer to the gate than the Dragonair lounge. You need to give them your RCC card and they will also ask to see your boarding pass even though it is not on UA. They will probably remind you to pay attention to your boarding time because they will not be announcing the boarding or departure of your flight since it is not on UA. Access is never a problem with a valid red RCC card.
Yes. They told me they check BPs to make sure you are the RCC member indeed. I don't think you can enter the TG lounge unless you're flying with a *A partner.