I agree with you that Plus options like this should be integrated into JJ2's ingame menus. Though I wouldn't try to display a full list of all possible game modes in the server setup window, but instead allow some scrolling mechanism like in the HCL and server lists.

Next, I wonder how you'd end up with a level without a tileset? I guess it would be possible to check for that... but still, what the heck?! After all, level downloads always come with their respective tilesets...

Also, there has been discussion about the boss music thing you mentioned. However, it will be made more or less obsolete by a new feature Violet has coming