Help/advice on changing a flight segment

I need some FT advice. Here's the facts and situation:

Mr. and Mrs. pinerd are travelling soon from SFO-ORD-YUL.
Mr. pinerd = UA Gold
Mrs. pinerd = UA Silver
We're currently on the same itinerary (which may get split with the PNR splitting...)

I just found out during my ORD-YUL segment, I need to be on an important phone call and thus cannot take that segment. I need to take a later segment that evening instead.

Some questions I have about what I need to do:
1) Can I do this SDC?
1a) If I do SDC, does Mrs. pinerd who is silver get it free since she's on my itin?
2) Should I split my segment and send Mrs. pinerd ahead and I take the later flight?
3) Should I do this ahead of time (i.e. now vs in 2 days)?

Any advice is much appreciated!