What are you talking about?
That oro has weakness against sharingan base genjutsu let alone tsukuyomi. This alone would deny all his chances against itachi.
Orochimau used the Curse Seal on Sasuke's neck as a medium to exist outside of Sasuke's body, but remained stuck to that medium. So aside from having no body, he also didn't have his arms.
Orochimaru was about to break Itachi's genjutsu. Orochimaru was weak and dying and needed a body, since he couldn't take Itachi's he had to leave to find another one.
Orochimaru appears to require eye contact to take someone's body. He stared into Itachi's eyes, stared into Genyuumaru's eyes, and stared into Sasuke's eyes. In fact, Orochimaru avoided eye contact with Sasuke until he engaged his soul transfer. In a fight to the death, Orochimaru has no reason to look into Itachi's eyes and will undoubtedly use snake sensing.
He still failed at doing that

and even if he was at full health we still don't know if he would be stronger than Itachi because he wasn't and it didn't happen. You're just assuming things, I'm going by the manga
Manga > fanfiction
You said it yourself, we don't know how strong 100% Orochimaru is in comparison to Itachi. You also say I can't assume things, and yet in your next breath say "manga > fanfiction" even though as you said the manga doesn't even say.
The funniest thing is, you have no idea what I'm assuming.
You don't seem to understand that there is no difference. There was no fight or brawl. He got shut down before he could do anythign. whether he was going for the kill or not wouldn't change him getting shut down by genjutsu in the start of the match
What you don't understand is that there is.
Orochimaru tried to use soul transfer, which requires eye contact, and got shut down.
Orochimaru has no reason to soul transfer if he's going for the kill.
Also, it's hilarious how you think Itachi "beat" Orochimaru by cutting off the hand of an immortal regenerating freak that giggles at getting chopped to pieces, and forgetting too that it was a weak and dying Orochimaru without killing intent.