There is no way that it works without breaking even more laws of physics than the game. So you're right, you can't gain momentum. Nor can you be deposited intact on the other side of the portal.

But of the options, the one you described seems the least likely. I keep telling you exactly how it wouldn't work, and rather than addressing the concerns you just say "no".

We can agree that you can partially enter a portal, so you can put your hand in and only your hand comes through the other side. So now tell me: how does your hand move out of the way for your arm to come through, without moving? Because if it moves, then it has gained momentum, which you've explicitly said doesn't happen.