Yes, you can redeem your RJ miles for an one-way AA/US flight and use your AA or BA points for the return. No problem at all. That's definitely what I would do before letting them expire. Any ticket you redeem them for is valid for one year. I don't know the intricacies of RJs program and if this is possible without fees, but you may even be able to book something now and change dates later.

Purchasing RJ miles may be expensive, but it's cheaper to buy a few topping up your balance (say, 1,500 to reach 20k which you could redeem for a ticket) than letting them expire and having to buy a revenue ticket
Combining or transferring points between oneworld programs is generally not possible with the exception of Avios which may be transferred between BA and IB (some restrictions apply).