Marinating rubbed brisket after smoking it

That would not be marinating, that is braising, which should produce a tender brisket without bark and offers another opportunity to add additional flavor to the meat.

Fat-cap up or down depends on your smoker and preference. In a vertical smoker, fat cap up was my preference. I rarely smoke a whole packer these days, opting for point/flat separation and lean trim.

Eric

Flat down , always for me    And the method you mentioned is called the 'Texas Crutch'  . Bad JU-JU , no Bark . . .

have fun and . . .