I'm assuming you're on UA56, which appears to have a lot of Polaris availability still (as compared to the later flight). IME the last seats to be taken typically will be 10DF and 12DF or possibly the "window" 10/12AL - at least 10/12DF meet your requirements ...

In terms of upgrades, if you're 6/7/8 if there's still 10+ seats available, you'll potentially clear before T-24, and hopefully you'll have some chance to grab seats you want. If you don't clear at that point, you'll be able to see the seat map and where you are on the list vs. seats available (actually you could do this around T-48). Then you can take off the upgrades, or take off two or one, if you don't want the risk of being seated separately.

One thing I've found is that if you are getting cleared at gate with a lot of open seats is that the gate agents do seem to make an effort to assign families with nearby seats (as opposed to just filling front to back in alpha order, which wouldn't always get that result if there are a few single seats).