I've read that Argonia was "conquered" by the Reman Empire and then later by Tiber Septim but I don't really believe it, so to speak. Sure they conquered it in name but neither of these empires even went into the inner swamps.

I have a hard time believing that Argonia was ever "conquered". I would suspect most of the tribes living in the inner swamps don't really care about what happens in the state of "Argonia" because I wouldn't suspect there's a lot of fierce nationality in a state divided into a bunch of warring tribes. They wouldn't even bother to leave their swamps to fight a bunch of Imperials that weren't going to journey into the inner swamps.

That's just my thought, though. Slavers have been able to ravage the place pretty well, so I've also read, but I was assuming Dres and whoever else was hunting slaves was just going around Northern Marshes, still not even scratching the surface of the swamps. What do you think?

The Saxhleel signed a piece of paper that the avatar of Sithis's soul showed them that essentially said "you're part of x Empire...yay...", and largely went on with their lives. The Cyrodiils were given access to the coasts and were allowed to do minor things here and that that largely amount to nothing in the long term.

Yeah, it was more a matter of them just becoming a part of the Empire, as opposed to being subjugated into it which would most likely not've gone down all that well. After all, during the Oblivion Crisis, the Argonians were doing so well fighting back that it was less Dagon's forces invading them, and more them invading Dagon's forces. He eventually had to pull out of Black Marsh because the Argonians were kicking his ass so hard. Argonians most definitely have a home field advantage in many ways, not a chance that Tiber Septim would've risked fighting that.