I can address my question in part after more investigation:
on https://issues.jboss.org/browse/JBPM-3582 - SingleSessionCommandService does execute the DisposeCommand in the finally block
on multiple machines accessing the same session ... there is an OPLTLOCK column in the SessionInfo table, which I assume is used as an optimistic lock mechanism. Does this confirm my suspicion that resurecting the same session via JPAKnowledgeService.loadStatefulKnowledgeSession() is safe across machines?