I've read something along the lines of it being because silver is a "noble metal" (or something like that) and vampires are unholy/foul creatures, somehow making them not be reflected by silver mirrors
I think open signups allow people to create an account without verification like email. I'm not sure about captchas, those might also count as a kind of verification.
A byte in this context always means 8 bit though, it has nothing to do with powers of 10 or 2. The prefix of K (kilo), M (mega), G (giga) or Ki (kibi), Mi (mebi), Gi (gibi) doesn't change the meaning of "byte".
It's of course because these creatures came from crypto currencies. They just took a while to mature to a point where people started witnessing them