Yes you can say "I thought of leaving in that instance" "I thought to leave in that instance" "I thought about leaving in that instance" you are describing what you were thinking "to leave when/if that instance were/should occur" "I thought to leave in an instant'" "I thought of leaving in an instant" "I thought about leaving in an instant" You are describing how you felt also what you were think of doing at the moment that instance occurred. "I am thinking to leave'" is not such a big error to many speakers who would probably add some extra information "I am thinking to leave this job soon/next week etc" "I am thinking of leaving" is more common natural and standard A lot depends on how you are defining the definition of "an instance" compared with an instant.

No, I would say I am thinking of or about leaving.