1. Seats on partner airlines will not go back into inventory immediately if you cancel the OW, and may never come back at all. Seats on UA airlines - you have a better chance of doing this.

2. If you have enough miles, you can see if you can "rebook" the RT and then only afterwards, cancel the one-way.

3. Your way of searching (daily) is why you don't see them. Most folks here have ExpertFlyer alerts, and they call immediately when they get that message. Sol it makes sense that you don't see availability on a once-daily look-up.


What you said below is your best advice (with one edit).

Sounds like my options are as I suspected: if I really want to be sure to get the flights I want, book two one-ways and skip the stopover.