Anybody can "buy" economy plus seats online. If you are Premier or higher you get them for free. I don't have status on UA but I've heard you can put your UA number in when buying the tickets to get economy plus seats and then after ticketed you can change your number to another airline and your seats won't change.

This is probably in the same boat as getting an upgrade on US and then changing your number to another airline. Some people have been reported that they were charged miles for the upgrade after the flight.

Envoy upgrade is based on availability. I would probably not spend $300 + 30k miles I would probably just book an outright award ticket depending on the level available.