I am wondering if MYM is going to have exclusive rewards for having pre-purchased it or not?
There aren't any exclusive preorder rewards. The deluxe edition rewards will become available separately after launch on April 4th (https://twitter.com/MeetYourMaker/status/1636802719104024576?s=20). The rewards if you own Dead by Daylight will be available until the foreseeable future (https://twitter.com/MeetYourMaker/status/1638567443051208706?s=20).
Looks like a no
There are exclusive pre-order skins
They are not exclusive to the pre-order, they are in the deluxe edition which likely will be available even after release.