Welcome, Allyt,
No reason to believe these are anything but Mikimoto pearls, with the box and documentation. These would be baroque Akoya, and appear to be in very good condition with high luster. They do need restringing and a bath, which you can learn to do yourself. Directions are here on the forum. Many of us restring our own pearls. The clasp is one I haven't seen before. I agree, the stones look like marcasite, very pretty.
The guarantee is very unusual!
Do you find the clamshell with the M inside stamped anywhere on the clasp?