No they can't. It's incredibly hard to amend the Constitution of the United States. That's by design. You need 2/3 vote in both the House and the Senate and 3/4 of all states to ratify an amendment in order to change it.
You can't be deemed "a dangerous criminal" without having actually been convicted in a court of law by a jury of your peers. And even then, there's appeals, pardons, etc.
> What good does the right to free speech do you if your speech is inconsequential?
Speech is never inconsequential. Even a one on one conversation can have a profound impact on another person. It's up to you to get your ideas out there and to compete in a diverse marketplace of ideas. That's the way it works.
> The US has repeatedly manufactured and framed political dissidents when it has felt politically threatened historically. Those rights you're talking about were useless to them.
Not generally, no. But sure, there are corrupt people in Government, no doubt. And that corruption has even been levied against a duly elected president of the United States (Trump vs the Obama FBI/CIA spygate). The existence of imperfection in the system is not proof of the system's uselessness.