Hi there,
I'm happy to help!
1) Which JR pass type would best suited for my above itinerary.
For your itinerary, especially if you're planning to explore the Fukuoka area, I would recommend the Sanyo-San'in Northern Kyushu Area Pass. It's the best option to cover both the Sanyo-San'in region and your desired destinations in Northern Kyushu.
Please note that this pass is valid for 7 consecutive days only. For any travel outside of the pass's validity period, you will need to purchase individual tickets.
2) What's the difference between sanyo sanin area pass vs. Sanyo sanin area with northern kyushu? Which one would be more suitable if I want to visit fukuoka area like Uminonakamichi seaside park? Can both JR pass covers the trip to the Uminonakamichi seaside park?
This pass covers the Sanyo area (including Osaka, Okayama, Hiroshima) and the San'in area. It will cover your travel to Fukuoka (Hakata Station).
This pass includes everything the Sanyo-San'in Area Pass offers, and also covers Northern Kyushu. That's the main difference.
I hope this helps you decide which pass is best for your trip!
Kind Regards,
Mariko