No he can't. Usually, only the originator of the information who classified it initially can downgrade it. On FOIA requests, each agency has a clearance process for releasing information. It goes through a panel, which decides what can and cannot be released.
Some levels of classification downgrade automatically through the passage of time. The bottom line is that the Pres and VP don't have carte blanche authority to declassify information. They are subject to the same rules as other members of the USG.
Yes he can.
That's the "usual" case that the OPR does it. But someone in his chain-of-command can also downgrade it
Yeah, that is the usual way to do it. But if the president deems that the info needs to be put into the public purview in a timely manner he can make it public. A president would not do this unless the circumstances are unusual.