And on the matter of this:
The main reason we're discussing whether MP checkbox should make it in and how should it work, is because it's kept in the j2l file unlike any other checkbox Violet decides to add. Or rather (since it's possible to change some stuff in the file structure afaik) because this checkbox is commonly used in level files already. As such, if there is a need for any kind of "advanced mode", there can be just another checkbox added and there is no point in changing the existing one. Editor mode checkboxes don't need to be kept inside of the level file, because that's rather pointless in most cases. On the other hand, drastically changing the meaning of the box could affect levels that already have it checked, depending on the changes. That's why I personally ask for leaving it as it is, because I use it as it is, but if there are better ideas I might change my mind. It doesn't seem there are any though.