In re: Trump's claimed authority to declassify. All presidents have the right to declassify information and to disseminate how they wish ...... up to a point. The authority to declassify derives from the president's role as commander-in-chief and chief executive. There are no statutes I am aware of governing this presidential authority, There are various executive orders concerning protocols for declassification, but executive orders are NOT binding upon the president. (Otherwise, Obama could have signed an executive order that his successor had to be honest and diligent). So, to reiterate, any president has broad power to declassify and disseminate as he chooses, but this power is NOT unlimited.
Let me provide an example: Franklin Roosevelt had the authority to declassify top secret war plans, however, had he called Hitler in late May of 1944 and told him to keep a kraut eyeball on the beaches of Normandy the first week of June ... he would have sabotaged D-Day and been guilty of treason.
Treason as a crime is something of an oddity. It is the only crime enumerated in the US Constitution (Article III, Section 3):
"Treason against the United States, shall consist only in levying War against them, or in adhering to their Enemies, giving them Aid and Comfort. No Person shall be convicted of Treason unless on the Testimony of two Witnesses to the same overt Act, or on Confession in open Court."
This is reiterated in Title 18 US Code, Chapter 115, Section 2381.
If Trump were charged with treason for his leak to the Russians, there would be two questions for the "trier of fact". (In most cases, the "trier of law" is the judge; the "trier of fact" is the jury, a panel of judges, or the sitting judge if there is no jury.):
Question 1: Is Russia an "enemy" of the United States?
Question 2: Did the leaked classified information provide "aid and comfort" to the Russians.
If the answer to these two questions is "yes"; then Trump is arguably guilty of treason. If the authority of the president to declassify crosses the line and gives aid and comfort to an enemy, then it is by definition, treason.