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.
Exactly, because it is almost certain that five teams from the Premier League will play in the Champions League next season since Arsenal was able to defeat Real Madrid in the first leg, I didn’t if Arsenal’s qualification to the semifinals needs to be complete before confirming that five teams from the Premier League will partake in the Champions League next season. Furthermore, if either Manchester United or Tottenham wins the Europa League, it means that six teams from the Premier League teams will play in the Champions League next season which will be a great improvement for the English Premier League.