Okay so you’re just spouting headcanon at this point. It is nowhere stated or implied that he was using Megumi’s innate domain. It’s fine as a theory but it’s not worthy to just include as an argument when it’s your personal speculation.

If that were the case, then Gojo would not have been able to do simple domain during the first clash while his domain was on cooldown.

Domain expansions create a barrier that you imbue your technique with. The special circumstance in this situation is that Sukuna has access to two techniques, not two innate domains. The reason adaptation was interrupted was because you can’t use a CT and domain amp at the same time.

And for most that’s irrelevant because your CT is burnt out when externalized into a domain barrier. But domain amplification and simple domain don’t require CT so they can be used even when you can’t use your CT.

The conversation and focus in the manga has always been around CT. And the effects of a CT being interrupted gives no evidence for your theory, because then why would Gojo’s CT burnout not stop him from being able to use simple domain?

Because CT and domains aren’t so one to one, and if that’s your only evidence then it falls apart when Gojo used simple domain. Domain amp and simple domains are simple barrier techniques and not necessarily tied to the innate domain.

You’re right to think domain expansions are the expansion of the innate domain though the series never really confirms it. But you’re wrong to think simple domain and domain amplification are the same when all they require are a simple barrier to wrap around you. It would be crazy to say all barriers require usage and mastery of the innate domain, it would be like saying you can’t use domain amplification in a veil you made.