@MattWho I deleted local NIFI state directory and it worked like magic :), I finally have my first 7 node secured NIFI cluster accessible using client cert.
Thanks a ton for your valuable inputs and detailed explanations without which I might have been still stuck, highly appreciated.
It may be beneficially to other users since this thread/topic was so long, if you accept each response that helped directly with getting to your final working cluster.
I am happy to hear that you are up and running. Now you get to explore the very granular authorizations/access policies NiFi provides within your secured cluster.