Article 2, Section A of the UCMJ shows that John Kerry was not subject to the UCMJ. He was in inactive reserve status and persons in this category are not subject to the UCMJ unless the offense that they committed occured while they were in a status subject to the UCMJ.
No. Reserve officers remain legally "Officers of the United States." They can be, and have been, involuntarily recalled to active duty to be court-martialed for acts done in an inactive status.