In this world, money talks. So being totally realistic, this has got some serious legs. It's backed by a massive American financial institution and the biggest football clubs in Europe. If they want to make it happen, it is a real thing.
But it's not without obstacle. For it to work, they NEED national and international football associations and leagues to agree to it. This is the main problem, because we're talking about financial compromises vs. sport.
The Premier League as an elite tournament and financial giant would be diminished if the ESL6 were to be thrown out. But it would also be diminished (probably more permanently) if they allowed 6 clubs to lock in a massive and permanent competitive advantage. It would confirm a 2-speed Premier League which goes against the foundations of sport. It wouldn't be sporting in the slightest and it would cause huge structural problems throughout the football pyramid.
If those clubs are truly that committed to this concept, then the only workable solution would be for them to withdraw from their national leagues and associations and form their own governing body outside UEFA and FIFA. That in itself is a massive legal challenge and would probably need some tweaks to the rules so that they aren't technically playing Association Football.
The clubs themselves would operate in isolation, like NFL franchises.
Players, managers and coaches are employees, so should be able to be employed by anyone (and therefore go to any club in any league) as long as they have the appropriate credentials. The clubs themselves though would need to choose between the status quo, or withdrawing from the game entirely and starting under a totally new structure which isn't part of the FIFA hierarchy.