I do not agree, but I do agree that they didn't really "fight", it was just Orochimaru trying to get Itachi's body.
1. You cannot say that Orochimaru wasn't taking it seriously. Against a ninja of Itachi's caliber, the first thing you do is TAKE it seriously. Orochimaru knows how powerful Itachi really is, so I am not sure why he'd hold back.
2. The timing of him saying that "he is even stronger than me" is implied to be at his full power with his arms. I think here you're just delving too deep into reasons as to why Itachi is weaker and came up with some way to benefit your point here.
3. It doesn't matter if he has multiple curse marks and can come back. He can't physically do it himself and someone has to unseal him. If he is killed and is just kinda waiting inside a curse mark, it still removes the immediate danger that he poses and hell, if nobody revives him ever then he STAYS inactive and is essentially dead. All Itachi would have to do at that point is hunt down anybody that has his curse marks (lol, since Kimmimaro, Anko and Part I/early Shippuden Sasuke were the only ones to have it that were actually somewhat strong, that's laughable) and kill them and essentially removes the threat of Orochimaru forever.
4. You said an Orochimaru without a body or arms got sealed right?
Okay, so Orochimaru has NO body or arms. Itachi was basically blind, dying of aids and had just fought Sasuke. I'd say handicaps are pretty even at MOST, but Orochimaru still got negged by Itachi.