You want speculation? OK, here goes. Yes, it will be easy. Or maybe not. I like to keep my bases covered.

If you check in at exactly T-24, I'd be surprised if you didn't manage to find two seats together, but I wouldn't count on it. We don't yet know whether United will take this opportunity to suddenly decrease the price for E+ buyups, but if they don't then I can't see there being nearly enough 1Ps and above to fill the E+ cabin too much. On a 763, something that helps is that the E+ cabin is nearly as big as the E- cabin. So I'd rate your chances as a totally made up 80%.

On your other inquiries, I've never changed at YUL but in my experience FRA isn't much fun if you don't have lounge access. Personally, of those three possibilities I'd take the YUL option (assuming nobody advises you that 2:20 is too short).