I have to say.. this hit a spot with me. We have been in this situation before (twice now). My DH father passed on when my DH was 14. So I never knew his real dad. When I joined the family his mom was remarried for 3 years prior to. My DH called his step dad by his first name. When we decided to start our own family. I told my husband I thought it was best our kids call him grandpa. He has since passed on (he was the best step dad a person could ask for). My MIL has remarried again (yes 3rd time) and now my kids call him Coleman and their kids call him gramps Coleman. They save the papa for their actual grandparents.
Now my GF's parents split 9 months after she married. Both her parents have since remarried, so these kids have 3 sets of grandparents, and how they handled it was, they say gram Nancy, gram Sue and Gram Betty. The guys are all called papa Dan, papa Bob and papa Earl. So why can't they call you nana then your first name?