1. IIRC he loses the trait when promoted. So don't.
2. Parking in blockade boxes is not moving. Trait doesn't apply there.
3. It is better to use your brigs getting supply. Frigates are better intercepters, but you can't just damage the Union economy so substantially to make the investment worthwhile. As brigs cost quite a sum, that they must generate as income before break even is reached, it is a worth consideration if you want to build new ones. The free brigs you get generate surplus from day one, which you would throw away if you would park them outside the blockade boxes. I never observed any difference in income with or without an admiral on board. I always recover their missing elements before sending them to duty though.
4. I never observed a difference in income depending on the number of stacks. Keep your brigs together to give a bit of protection against annihilation (in combat). Same is the only reason to provide them with an admiral, imho.
5. Tattnall, imho, is best used to sneak the initial fleet out of Richmond (hopefully withoput killing him in the process) and in later game, when you lost river supremacy. You can still move fleets around with his trait and "avoid combat" turned on, mainly to move supply and troops around with transports attached to your ironclads.