flying with infant, no seats next to spouse

I have a 4 hour flight with an infant in lap in a few weeks. I purchased this flight in Jan and it was FULL. No seats next to each other except in exit row, which I couldn’t book because of lap infant. This is the only flight that day. The best I could do is one window (bulkhead) and one aisle 5 rows behind.

I’ve called United and I check every few days for the last 2 months for adjacent seats but no luck. I am hoping one of the adjacent middle seats will want to switch so we can sit together as a last resort.

Does anyone have advice on getting adjacent seats? Customer service told me to routinely check the app for an opening and that’s all I can hope for.

Any advice on bribing a fellow passenger to swap?

Thank you!

TO ADD: We’re both in econ plus and have gold status, and of course I would only ask nicely for one of the middles to switch to our window or aisle (both arguably better seats).