mystery sterling mid century? modernist necklace

I bought this gorgeous necklace second-hand from someone who didn't know its history. It looks mid-century modernist to me, but it's not signed and I'm no expert. It's forged sterling with a crazy-lace agate drop pendant (no signature) and high quality metalwork on the four side pendants. (There were never stones on the outer pendants; the "craters" are metalwork.) It's tarnished and filthy (the craters are half full of grime), but beautiful even now.

Alas, when it showed up, the clasp section (and any signature) was missing. It's been replaced with an ordinary piece of (silver?) wire bent into a hook and eye at either end. This is awful! It's hard to fasten, insecure, and twists the links so the necklace can't hang properly. I feel like Cinderella outfitted in a stunning ball gown and Crocs. And, of course, if there was ever a signature, it's gone.

So three questions:
1. Why in the world would anyone do such a thing?
2. Can anyone identify this gorgeous piece?
3. Is there any way to source a more appropriate clasp? I could insist (boy, could I) that the seller take it back, since it isn't wearable, but it's so beautiful that I'd rather try to fix it.

Thank you! All advice appreciated.

Susan