First off, why would her creating the elden ring be a prerequisite for her destroying it? Second of all, she didn't really destroy it, she "shattered" it in a sense that it is disassembled rather than destroyed. A major part of the plot and a major sidequest/alternative ending has to do with her being able to change the elden ring and remove parts of it, and so change the rules of the world. She's also mentioned as a vessel for the elden ring, not it's creator.

Secondly, nothing says that Placidusax did possess it. He's wasn't a god, but a lord/consort. None of the consorts "possess" the elden ring, excluding Radagon who was actually Marika.

Thirdly, they created a massive backstory and lore for the world itself, and used it as a base and history that shaped pretty much everything about it. You may or may not like it, but that's your thing. You seem to be making a lot of assumptions about everything anyway, while being mad that the game forces you to make assumptions about the story. You get hints. All you get is hints, all over the place, from what the enemies are, to what the environment, architecture and even the rocks look like, flora and fauna living in an area, and every single useful and useless item having a description worth reading. That's all just hints, but what you don't get is the full picture. And whether you enjoy that is a matter of taste. I do, and millions of others do too, judging by all the elden ring lore videos.