Since Grey Havens updated the wiki software, categories are a bit broken. For example, as you said, regular pages can't be added to categories (at least I don't know of any way that works). But images work for some reason. You can see this phenomenon on any recent category page. (like https://wiki.spiralknights.com/Category:Late_Harvest-Themed)

I think the database error started showing after that update, too, but I'm not sure. It seemed to happen only on certain pages and didn't seem to impact anything (after I refresh the edited page I get a message that my edit was saved successfully), so I learned to ignore it. But it makes sense to be connected with categories, I admit.

It's possible that Grey Havens don't know about these issues, so maybe it's worth to send them a mail or support ticket or something.

Yes, that should be the proper format for linking categories.

You can't delete pages yourself, you need to ask admins for that. But if you accidentally link a page to a nonexistent category, just link it where it belongs and I guess you should be fine, because the category page in some sense still doesn't exist, there is just a list of pages that were linked to it. Although, regarding the issue I mentioned above, I doubt it will have any practical effect right now.

I wrote a really long text speculating whether it was good or bad decision to remove status categories from the color pattern list, but it doesn't affect that many items, so I guess it's not that scary change after all. Still, I'd rather have them at least as some kind of side themes and wouldn't remove them entirely.