I would not think a Congressional resolution proclaiming him an American citizen would change this analysis much, since it appears he can be tried for treason even as a resident alien?
(I think also that a good defense attorney could argue all day about a Congressional declaration of a fact as “law,” where that fact is demonstrably false... and where it were to be used, perhaps, as an essential element in a criminal conviction... raises all sorts of fun questions... ha... but at any event, it does not appear necessary to reach this ..)
I remain very open to any corrections or improvements on my earlier posting which is to be considered as merely a preliminary first stab at the “can a treason charge stand?” question. Perhaps there is a lawyer out there with the time and resources and expertise to really research this for us? Thanks!