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.
Do you mean six teams are going to qualify for the champions league since if two more teams are added to those in the first four positions, then the one that won the Europa League and the one that won the champions league will make them the fifth and sixth teams to qualify for the Champions League next season.
The champions league can only get seven teams if all what you said will be the case and five teams are allocated initially from the premier league to represent them in the champions league. The premier league is a very tough competition though, I still feel the two premier league teams left in the UCL can do something spectacular we may not be expecting in this champions league.