Does anyone know the pre-order level can be unlocked later in the game or exclusive? I'm tempted to preorder the special edition that include Demon and Assassin pack though if these can be unlocked by spending gold in the game then i would get the standard one since the difference is the $16

I'd sincerely doubt that it'd be unlocked with gold. But the basic question is no one will know the answer to your question
until the game is available for purchase / if a game developer / publisher responded. And if it was answered the question could
potentially have an adverse result on preorders, so it won't be answered.

With that said:

The hitch may be simply that purchasing them = the level/boss premade + also includes them as parts/usable pieces
for creating your own custom dungeon runs.

So, it may be a case where the user who has purchased the preorder content will end up hosting it online.

This would be similar to what has been done with Little Big Planet (Metal Gear Solid DLC content) and
ModNation Racers where people very quickly reproduced Kratos, Drake, and other pre-order mods.