Can you use the files you get (DB files) for modding, or do you have to wait for release date?

You'd need to wait till release date, because I doubt that preordering gives you the .esm and all that.

Even if/when it's preloadable, the files are going to be encrypted by Steam. Gonna have to wait till Feb 5th.

You wouldnt need the esm to use the assets if you had the BSAs. The limiting factor would be any mods using the assets would be limited to people who also had the BSAs inorder to use them, as destributing the assets with your mods would be illegal,