1) Will our bags be checked through, or will we have the option to go either way? Presumably we will have to walk our checked bags through US Customs.
2) When we get off the plane, will we be shunted straight to passport control/Customs or can we stop at the United Club?
I can't say whether you'll be able to check through, or be able to choose to short check to GUM. However, there is no US customs. If your bag is checked to GUM, you'll proceed to Guam customs after US immigration. If your bag is checked through, you just go through Guam customs with your carry-on.
The GUM concourse is now divided along the whole length into halves, and all arriving passengers (except from SPN and HNL) will be directed to the half that goes only to US immigration.