It's only if United wins Europa League that only the top four clubs on the table at the end of the season will qualify to UCL.
No.
In the case Manchester United wins the Europa league this season, they will qualify for the champions league and the 5th placed team of the premier league will qualify for the champions league too.
It's even possible that 7 english teams will take part in the champions league next season. That will happen if Aston Villa wins the champions league this season without ending the premier league at top 5 positions and Manchester United wins the Europa league.
I thought English premier League clubs that qualify for champions league games are the top 4 positions on the table, while if a winner of Europa League title automatically qualifies for champions league tournament if the club doesn't qualify off the top 4 in EPL table. Although, the qualification criteria for champions league is changing depending on the strength of the league especially EPL, laliga, bundesliga and serie A, these top four leagues are always choosen to participate mostly in the champions league games, I think so.