You're mixing the two lists together. The A list has no u-21 requirements.
The A list is the 25 man squad, where you need 4 club trained players and 4 association trained players to name a complete squad. For any requirements you can't meet you have to leave blank spaces in the squad, leaving us with the ability to name a 21 man squad as you say because we have no club trained players.
U-21 players can be registered for the competition as extras on the B list, but in order to qualify for the B list they must have spent two full consecutive seasons at the club, of which we have none.
So no matter what we do we're stuck with a 21 man squad for the Europa league. I think the only two players in the world, who have anything about them, that we could sign to extend that number would be Tom Cairney and Liam Cooper.
Probably some of our development squad will qualify for the b list but due to the fact none have them have ever made a first team appearance and the assumption that they're all ****e, I think we can discard them as any real additions to the squad.