Ah yeah I think you're right. Just to clear this up once and for all...I may be missing something, but I have us as the first team in pot 3 if United lose. Therefore, we would only need to pass one of Sevilla or Napoli and would need one of them to lose, not both.
Here are the club rankings, taken from the UEFA website.
So, there are 19 teams ahead of us in the rankings. Of those, 6 have qualified for pot 1 as league winners, and 4 definitely haven't qualified at all. That leaves 9 teams. Of those, 6 have definitely qualified already, and will definitely be in pot 2. These are Barca, Atletico, PSG, Dortmund, Man City and Porto. Then there are 3 teams left who could take the last 2 spots in pot 2 ahead of us: Sevilla, Man U and Napoli. Sevilla will need to win a play-off to qualify, and Man U obviously need to win tonight. Napoli can qualify in two ways: if they win away at Sampdoria and Roma drop points at home to Genoa on Sunday they'll finish second in the league and qualify, or if, as is likely, that doesn't happen then they'll finish third and have to win a play-off.
For us to get into pot 2, we need no more than 1 of those 3 to qualify. So chances are, we'll be left hanging until the play-off round is over until we know which pot we're in.