PvE and PvP are already classified separately, so thats hopefully just a bug on Blizzards side (or possibly user stupidity in using it?)
Groups don't extend past their own region, so NA+Oceanic are really the only with the region requirement for world bosses, otherwise its really not required information (would never see a NA group in EU, for example)
Could possibly use user-readable tags for the stuff that remains, that makes it less confusing for everyone.
Quite a bunch of your requirements are sadly not possible, since while you can write a replacement UI, you cannot get any extra information that the functions don't give you, or add any restrictions or anything like that.
For example, you couldn't create new categories, as Blizzard defines which categories exist and which do not.
Practically, you can take the data the group finder gives you and show it in nicer ways, maybe make the search a bit more sane, but sadly any actual new functions are rather limited.
There are no scenarios at level 100, which is why its not part of the group finder.