Can Nibbana be experienced by believers in 'rebirth-reincarnation'?
"Therefore, can the thought of or belief in 'rebirth' in the mind
  result in Nibbana?"


No, it can't.

In the same way, the thought of or belief in "non existence of rebirth" in the mind, can't result in Nibanna.


  "Is the belief in 'rebirth-reincarnation' not a type of 'becoming' &amp;
  not a 'thought formation'?"


It is a type of "becoming" and a "thought formation".

In the same way, a belief in "non existence of rebirth-reincarnation", is a type of "becoming" and a "thought formation".

When a thought or belief arises in a being due to ignorance, that being clings and craves on that thought or belief. This clinging and craving will not result in Nibanna.

When a thought arises in a being due to wisdom, that being does not cling nor crave on that thought. This absence of clinging and craving will result in Nibanna.

A belief is something that is accepted as the only truth. Thus, no belief can ever arise in a being who truly destroyed ignorance. That would be impossible. The one who destroyed ignorance, dwells in wisdom. He is the knower of the truth. How could the knower of the truth say "this is the only truth", when he knows "all truths are non truths"? That would be impossible. That's why the knower of the truth stays in the middle.