Is there an official order of priority with these at-the-gate (or in-the-lounge) upgrades?

Is it purely first-come-first-served or is it based on EB (*A) status and/or ticket class?

I have an upcoming IAH-ORD-ARN-OSL trip in Y+ (with UA on the IAH-ORD segment) so I will not be able to request an upgrade on ORD-ARN until I'm at the gate in ORD about 1.5hrs before departure... So I'm guessing slim odds for a business class upgrade?

I'm directing all the earned miles for these flights to my UA account, but I have some orphaned soon-to-expire SK points that I was hoping to use for the upgrade. I trust there is no issue in deducting SK points for an upgrade but crediting earned miles to UA?

On the return, can I request an upgrade on the ARN-ORD segment when I check-in at OSL, or do I have to wait until I'm at the gate in ARN? (probably only 40min prior to departure)

And finally, any thoughts on the best seat assignment in Y+