Yes, you get two RDMs for every 1 BIS.

Yes, you can use RCC (and other, nicer *A lounges) in the US on same day int'l itin (though I think you only get RCC drink chits at the airport at which your overwater segment originates; YMMV)

1Ps don't get SWUs. If you get one gifted to you, you are higher on the waitlist than 2Ps. Also, for op-ups (which do happen).

There are two other benefits I like: reserving exit rows, and the 1P line connects to US agents 90% of the time.