If he failed to memorialize the declassifications and follow EO dictated protocols as President, then,, as a private citizen, if challenged on this point, he will need to prove that he declassified the docs. Given that he didn't follow protocols, which would provide the proof needed, say, in a court of law, they are effectively still classified. The point is, he, as a private citizen, cannot claim they were declassified when he was president if he can't prove it (should he ever need to).
This is the whole point of following protocols,so that everyone in the intel community will know and the courts, as well, should it be a point of legal contention.
Yes, he can. If you contend otherwise, provide a cite.
I contend otherwise, per
10 CFR § 1045.155 - How is matter marked as containing RD, FRD, or TFNI declassified? | Electronic Code of Federal Regulations (e-CFR) | US Law | LII / Legal Information Institute (cornell.edu)
Summarized here:
Government Classification and the Mar-a-Lago Documents | Brennan Center for Justice
Except for Congress, any "issuing authority works for the President.
See above
Also note that in the case of being prosecuted for violation of the Espionage act, such does not depend on any classification regime. DNI Damage assessments are more likely to be focused on.