In short: I completed reading the manga, and I think our fellow authors
should know this:
1.) Ginzuisho is a part of natural order. It is probably as old as
Universe itself (there is a confirmed fact that it is older than our
Solar System). If we assume it was created, then most probably by
Allmighty himself.
2.) Ginzuisho is just a non-standard, very powerful, sailor crystal
(these are very common things in this galaxy). A sailor crystal is just
a very powerful star crystal (star seed in anime), allowing to link to
a planet. Linking is performed when someone becomes a Senshi first
time. There are beings who has a sailor crystal but isn't a Senshi (not
linked to a planet). These include: Luna, Artemis, Diana, Phobos,
Deimos (all from other star systems that didn't need so many Senshi).
Reference in the anime: Episode 193. Sailor Tin Nyanko rips out Usagi's Star Seed. And what do you think emerges then?.. Of course, our old pal Ginzuisho! (in its "crystal flower" form). The baddies immediately call it "Supreme star seed".
3.) The Senshi are a part of natural order. They aren't a result of
some technomagic research. They are born natural way throughout entire
Galaxy.
4.) Tsukino Usagi is a manifestation of Silver Crystal. Without it she
wouldn't exist. Her soul is linked to it, not to her body (same for any
Senshi). Since the sailor crystals are ethernal and indestructible, it
allows them to reincarnate again and again.
5.) Ginzuisho is not a material object. It can be carried by the
spirits of the dead.
This was done by the spirit of Hotaru (when her body was occupied by Mistress 9) and the spirit of Queen Serenity (when she gave her power to Serenity Jr's Star Crystal)