I really doubt Congress is going to start forming factions
It already has factions. They couldn't even pick a Speaker of the House without causing a ruckus. Republicans are united around Trump, and only Trump. They can't decide if the Speaker should be more libertarian, more Christian Nationalist, or just straight up MAGA faithful.
Your scenario also assumes that support of Congress is needed. The White House already has their Gestapo, ICE, as well as the actual US Military. They’ve broken Supreme Court orders, some of them unanimous 9-0 decisions.
Only because everyone says "let Trump do whatever he wants". Nobody else can take up that mantle. Someone will try, probably Vance. He's an idiot who doesn't have anything like Trump's ability to appeal to a certain kind of person, and will fail. Hard.
To assume that any legal action can stop the regime is to utterly ignore reality. There was nothing legal about George W. Bush’s invasion of Iraq or Afghanistan.
They still had to play the game where Congress authorizes it. It was only "illegal" in the sense that international law forbids it, and international law isn't quite as clear cut about enforcement mechanisms. It's more like a series of agreements between nation states, and those agreements can get tossed. Especially when the guy with the biggest hammer (the US) decides they can be tossed. Nobody was going to do anything substantive against the US for that.
The Constitution has been shredded in every sense but literal. The only way one can believe we have any protection is to presume that “it would never happen here”… in the face of “it” happening here.
I'm not assuming anything about the Constitution whatsoever. At its most basic, politics is coalitions of people who try to achieve certain goals. Voting and a constitutional republic are just means we've traditionally used for structuring that. Coalitions form without any other structure being present, and can fall apart when a central leader goes away. In a scenario where Trump dies and Congress is still around, Congressional Republicans will fight each other trying to be on top. No reason to assume they will form around a new leader at all.