Looks to me like the fic is a series of one-shots with a unifying theme of Valentine's Day, but otherwise no connection. The author's native language probably doesn't use articles (a, an, the), hence the lack of them.

The third chapter does look like a Steven Universe crossover to me, too. Fusions are a thing Gems and part-Gems (i.e. Steven) can do, including with humans (though it takes a lot of practice); the resulting being takes a new name and is something greater than the sum of its parts, at least in a stable fusion. Unstable ones split apart again pretty quickly. Permanent fusions are frowned upon in Gem society, but one of the main characters in the show is one; they sometimes act as a metaphor for a stable, loving partnership and/or for healthy mental balance.

The core of each Gem's being is a literal gemstone that appears somewhere on their body. The "triangle for an eye" thing seems to be a reference to that, except the gemstones don't fuse, they just migrate to their usual location on the new being, so IDK.

The four Diamonds are top-tier Gems who rule their society, and we know who all of them are by the end of the show, so casting Bill as one of them is a bit rich. There is canonical precedent for the "former Diamond" thing, but it's a major spoiler and wouldn't help the chapter make any more sense, so I won't get into it.

I don't know if those bits of explanation help at all, anyway. {X D

~Neshomeh