1) I'd go for F unless the 20k miles means A LOT to you and you have other clear uses for it. Trying OZ F seems worth 20k to me, plus you get to experience a TG F departure ex-BKK. Now, IIRC the BKK-HKG flight in F is an 8am departure, but I still decided to show up VERY VERY early at the airport when I took the flight a few months ago so I would have time for a massage

2. United agents are known for making up their own routing rules. As long as you successfully ticket you're fine. I've been told that I couldn't do a stopover in PEK on the way to BKK "because you have to follow the overwater carrier (in this case, OZ)'s routing rules" even though OZ explcitly files their fares with PEK stopover provisions. They wanted to restrict me to an open jaw, and have me swim to Thailand. Hah! Just called back and got what I wanted.

3. Since i don't know your dates it's hard to suggest flights, but is ICN-LAX or ICN-ORD available on your return in OZ F? You could fly HKG-ICN to connect. Or if travel is before the end of March you could search for NRT-ORD on NH. (F availability pretty much disappears once we leave winter.)