I've seen clasps very close to this one before; with Mikimoto's (lack of) tracking and openness about the history of their pieces and clasps, it wouldn't surprise me if it's a genuine Mikimoto clasp. I'm not a trained Mikimoto expert, but their clasps tend to have their single pearl on it, of course, and some sort of symmetrical design, often nature-inspired - a leaf motif, for example.
Mikimoto necklace clasps had more tendency to be all over the place. 18k necklace clasps are the more standard option found these days, but I wouldn't think that 14k should be considered a warning sign.
However, to be frank, the pearls on this strand don't look well matched enough for true top Mikimoto quality - perhaps this necklace was an A1 grade, or something from the "branch off" lines like Blue Lagoon or Sea Magic. The pearls certainly don't look bad, but consider how much you're paying for possible Mikimoto here.
I also looked at your bracelet thread. That one I'm more confident about. That blue, lizard-skin patterned box is the standard style for regular Mikimoto items (i.e. not one off pieces) bought from retailers who carry Mikimoto. The bought-direct from Mikimoto boutique store packaging is different from the blue.