It was. Even Davis referred to it as a rebellion at a speech in Mississippi in 1863. The fact that you disagree with the commonly accepted definition of rebellion doesn't mean anything.
It was secession, not an effort to take over the existing government. It was a continuation of the process the founders asserted as a right in 1776.
Rebellion is defined as open, armed defiance to an established government. It doesn't require wanting to take it over. Unless, of course, you're making up your own definitions now.
I don't see your relevance for Article IV. Guaranteeing a Republican form of government would require Federal involvement in the enforcement of the 14th amendment if States do not enforce it themselves.
Maybe if you read it? "The United States shall guarantee to every state in this union a republican form of government, and shall protect each of them against invasion; and on application of the legislature, or of the executive (when the legislature cannot be convened) against domestic violence.
These rioters are denying the civil rights of other people, and these are clearly 14th amendment violations.
Of course they are.
I do not define my understanding on the basis of what other people assert. This is the fallacy of "argumentum ad populum".
Rebellion is defined as open, armed defiance to an established government.
The previous government had been deestablished in accord with the right asserted in the Declaration of Independence.
Refusing to accept the lawful result of people exercising their right to independence in according with established law, is "rebellion."