But it hasn’t been established legally and officially that he’s a usurper.
It's been established that he never "qualified". That by itself is enough.
Not one person in Congress will attest under oath to the legal authenticity of his birth certificate or that they have even seen such. They are commanded by the Twentieth Amendment, Section Three to have done so and if he was found to not be eligible, appoint an interim President who "shall have qualified". This wasn't done. A "qualification" did not occur. That there is any question about his being eligible still exist tells you it never happened. That alone is enough to prove we have a reasonable doubt about his legal status and probably a usurpation of the Presidency. The destruction has gone on now for too long to not challenge his legitimacy.