To answer that question, I think there are just a lot of different ways to format YAML frontmatter. I don’t know if any of your examples are incorrect, but there is definitely more than one correct way to do things.
Obsidian recently implemented “Properties”. Before that it supported YAML frontmatter. (I think it still is YAML, but now more powerful. I’m not 100% sure.)
So you’ve likely seen many different types of examples because Obsidian has been evolving very quickly in the few years since it was first released. And best practices were (and still are) being figured out. And now Properties is a huge fundamental change, so it’s likely best just to do as solarboi recommended, and follow the current best-practices for Properties.
Well, in Obsidian technically yes it is. 
If you put a hashtag before something, then in Obsidian it will be recognized as a tag. If you don’t want it to be a tag, you can escape the # by using a backslash. (Backslash escapes other special characters too.)
This \#Example will not be a tag.