It seems like the “best” solution is for the OP to book a separate O/W from SFO-EWR, but is worried about UA flagging the O/W and canceling one or both PNR’s.
Presumably, the OP’s MP # would be the mechanism that UA would use to flag both itineraries as belonging to the OP.
So why not book the SFO-EWR O/W without any MP#? Then, after the O/B is flown, cancel the return, and then the OP can add their MP# to the O/W reservation. This might result in the OP losing opportunities regarding upgrades etc., but it would allow the OP to fly their desired routing at the lowest cost while eliminating the uncertainties that come with relying on SDC and stand-by.
Thoughts?