On my phone and short of time so can't break this down comprehensively, but...
City have only done better than United in the last 4 of those 7 years, which coincided with United signing Falcao, Pogba, Di Maria, Mkhi and generally fewer British players; until that point, Cleverley, Evans, Young and Welbeck all featured prominently.
The City team that won 6-1 (the score line which effectively won the title via GD) had 5 English starters.
Leicester generally had 5 British starters last season. Spurs generally accomodate 5, and are overachieving in relation to financial outlay.
You say the successful English teams bar Ferguson's United haven't necessarily embraced the same pro-British principles; that's true, and that's why none of them have been able to replicate the sustained success or establish a dynasty, which given their vulgar investments in an attempt to, is massive underachievement. Tottenham have, and have taken a quantum leap from mediocrity to most complete squad with the most legitimate upward trajectory in English football, on a humble budget.
The Real Madrid point you're trying to make is a bit strange; Zidane's predecessors also had Modric, Ronaldo, Marcelo, and failed. So this suggests that the reason he WASN'T 'sacked by February' is the functional Spanish elements of the squad he's introduced that glue the team/squad together. They're going to win the league and could win a second consecutive European Cup. His tactical input is obviously crucial to this, but the most noticeable difference between him and previous coaches there is his advocacy of Spanish integration.
He will sign De Gea this summer, adding another international to the collection, add Theo Hernandez (a player of Spanish heritage, football development and upbringing) and bring Marcos Llorente and Jesus Vallejo back from their respective loans; he's clearly wants them to carry a distinctly nationalist identity, like Barca, Bayern and Juve do.
Big part of United downhill was signing a local manager just because he's British and having none of the credentials those pesky foreigners have. I don't see how Pool or other top teams profited of signing Lallana, Stones, Jones, Smalling, Sterling, etc. for crazy fees.
United problem was not that they've signed top players but LvG and Moyes 'philosophies'. Di Maria didn't do too badly at Real neither at PSG, yet under performed massively at United although off with a bright start.
If you think that with Cleverley, Evans and Welbeck we'd be better under Moyes, then I strongly disagree. One of the best players Moyes signed was Mata and he has been probably the most creative outlet we have had in the last couple of years. Should we continue to play Rooney because he's English, yet constantly stinking up the joint?
I don't really care what nationality our players are, the main point is them being good enough. Are Gibson, Cleverley, Welbeck, Rooney, etc good enough to start week in and week out? I don't think so.
All English clubs that have done something in Europe have moved away from the British approach and embraced foreign ideas. I can't think of a single British manager for example (apart from Fergie of course) who had a particular great European record managing a local team. Spurs have won diddly squat lately and are regularly humbled in Europe, lately by a Belgian team where usually top team from England send players on loan to gain experience, who consequently lost to another Belgium team on 6-3 aggregate and finished 4th this year in Belgium behind colossus Zulte Waregem.
Spurs are hardly a shining example in Europe tbh.
I really don't see the Real Madrid point. Real will most likely finish on around the same points in the league like they did in the previous two years when they finished behind Barca (around 90-93 points), and they won the CL and made it to the SF's the years before he took control.
He inherited essentially Ancelotti's team that won the CL against Atletico 2 years before. The so called introduction of Spanish players is really no different to what his predecessors did before him. Zidane showed Jese the door a 24 years forward with around 100 apps for the club at the time and introduced Lucas. Nacho is 27 years old defender who already was a rotational player and featured in about 20 games per season for previous managers. He featured a lot this season because Pepe is 34 years old and suffered fractured ribs, calf and hamstrung injuries that kept him sidelined for half of the season. Moratta has played around 5 full games this season and was on the bench in moments when he was in better form than Benzema.
I think you are putting too much into this nationalist identity as teams like Barca, Real, Juve, Bayern buy the best players they can get in the respective positions, rather than go national. If they happen to be with the same nationality it's just an added bonus.