I would skip attempting air layer anything. With that advice, People are advising you to run before you can walk. It's not worth the effort and anxiety at this point. Even if you are successful with that operation, you will wind up with more trees you don't know what to do with.
Concentrate on the main tree.
Here's what I'd do. Simply keep them alive through the summer, fall and winter-which might not be easy with that root bound elm. Next spring, repot and root prune both. Work on getting the roots settled next spring, THEN move on to more drastic measures like heavy pruning, the following year.
This sound relatively simple, but first time trees are usually challenging. Simple steps, learning to water, learning to understand WHEN to water, seeing how trees grow--timing of new growth, how it elongates and when you can trim it are all mysteries to you at this point, much less how to air layer. Learn the basic steps, before trying to tap dance...