I've noticed that there are a lot of references to the Aragonians being created and at least partly controlled by the Hist, so my question is- can the Aragonians make any independent decisions, or are they completely controlled by the Hist? Also, if they are completely controlled, would it be possible to break free?

Yes, they can make independent choices.

Source material: The Infernal City + every Argonian we've ever met in-game.

It likely varies from argonian to argonian. But I'm pretty sure that they all have at least some degree of autonomy.

Yes, they can. On the one hand the power of Hist whispering fades when they are far away, and even when they are at home, they can resist, thay're not like drones. The TES novels are great in this department, they tell a few things but still leave room for ambiguity and imagination.

Nice, Argonian question! Smiles gleefully and begins to type.

First off, it's Argonian without the extra a after the r. Just wanted to point that out.

Yes, the Argonians do have free will. If they didn't they wouldn't have names or personal opinions about other people in the games. The Argonians are capable of independent thought, can have bias towards any person (for example, some Argonians have no problem with Dunmer, which their general population absolutely despise), and have their own memories and dreams.

The Hist do have control over their minds, since they are their creators.

First off, thanks for pointing out the spelling.

But, are you saying that the Hist are able to control the Argonians, but they usually don't? I guess that would make some sense

I wouldn't say that the Argonians that we meet in game are a good example since the Hist could be directing them to do whatever they do

You can play as an Argonian.

Control is an incorrect word, Influence is perhaps, more accurate. The Argonains and Hist share a symbiotic relationship. The Hist give the Argonians purpose, identity, and life. The Argonians are expected to protect Black Marsh and the Hist against any and all outside threats. When the Hist is threatened, they can "pump" the Argonians up with... I guess you can say "metaphysical steroids."

But direct control? No. An Argonian in Black Marsh can resist. But they choose not too. Because the Hist greatly benefit them, and they in return, benefit the Hist.

Mere Glim did not resit when the Hist of Lilmoth ordered him to sacrifice himself to Umbriel.

It may depend on how closely linked through life the Argonians are to the hist. Many were not afraid to get all cracked out on Hist-sap, go nuts, and hurl themselves into a violent death in a all out assault on Dagon's forces. At the same time, not all went back to Black Marsh anyway, it probably varies from person to person.

Sounds like Mere Glim was very loyal to Lilmoth. Argonians are known for showing irrevocable loyalty to whomever they pledge loyalty to.