I booked BA flights for my parents and they will be arriving from LHR to IAD. My brother who is in Washington DC got a new job few months after making this reservation and now my brother is moving to New Jersey (Original plan was my parents to visit my brother, but with this unexpected move to New Jersey, we are in a dilemma). I live in SFO and I wanted them to visit SFO and possibly make changes to my parents flight and wanted to check and see if this is possible?

My parents are flying from BAH to LHR and then LHR to IAD in BA 217 A380 in Business class.

1. Scenario # 1
I have lots of United Airlines miles that I would like to use for them to fly from IAD to SFO in coach.
When they arrive in IAD, they would ideally need to do all immigration formalities which also includes rechecking the baggage.

My question is: Since they fly business class, should they paying for baggage for the United segment if it combined from the home airport?
Or can their baggage be checked all the way to SFO, so that after completing their immigration check, they don't have to pay for the baggage?

Since this scenario involve alliance change, is it even possible to make such change?

2. Scenario # 2
Is there a possibility to change their flight to fly directly to SFO, thereby we can pay the difference of the air fees? In this case, what are other fees involved and what other things to consider? Is there any change fee by the airline?

Thanks.