It looks to me like this is a happy medium between more formal and more colloquial (although there are probably ways to say that in slang, but I’m not sure your question is about that). I wouldn’t venture in “n’eusse-t’il pas compris” territory – maybe there is a subjunctive possibility (!) but I don’t want to go there. One could say:

si simplement que tu te demanderas d’abord: n’a-t-il pas compris?

In which there is less uncertainty.

One thing though:

si simplement que tu te demandes d’abord : n’ait-il pas compris ?

Doesn’t work. It would have to be, maybe (but the agreement of the tenses feels wrong):

si simplement que tu te demandes d’abord : n’avait-il pas compris ?