I read a hint somewhere that this genuinely might not be the case - the Qatari world cup is such a farce now I'll believe anything!
I suppose it would be upto each league to organise themselves. The champions league has already been suspended for that period, there's no chance the PL would play on. Think of the advertising revenue that would be sacrificed by competing with the world cup.
Yes I haven't seen any suggestion that the top flight leagues would continue with youth team players while the World Cup is on.
It'll be interesting to see how many international players in the Prem develop "injuries" that preclude selection just prior to the Qatar World Cup. I really hope not but I can see it happening, with half the season and various cup commitments to come after the WC.