Agree with others, most likely explanation is someone bought a ticket while you were searching. Dynamic pricing applies to award tickets too, and an open jaw/return should not price differently from two one ways.

On your second question, these are all UA metal flights so can be booked with pooled miles. It takes a couple of days to set up a pool and move miles over. But like I said, I don't think the issue is not being able to book as one itinerary. Your original strategy was sound, and in fact preferred.