I'm really glad you liked it. The issue with separate or individual lives was actually how the initial version of the co-op mode worked, but it made the game boring if one of the players lost all their lives too quickly. The idea is for players to collaborate with each other. Besides, if lives were individual, if you earned an extra life, it would only be for you.
Regarding the DivMMC, I'm currently working on a version with DivMMC support—specifically for esxDOS. This version automatically saves progress and settings. I'm currently fine-tuning the hardware detection. An exclusive NEXT version will also be coming out soon.
Unfortunately, I don't own a NEXT or a DivMMC, so I'm testing everything using emulators. I will try to look into all the specific cases you mentioned.
Thank you so much for your comments and feedback!