Dav3y, lets get something settled. Are you talking about the CBC/MAGTECH extruded all brass cases? If so, they must have larger diameter wads than standard 12 GA. wads for plastic shells. The brass is so much thinner than any plastic shells that the standard wads are much too loose/small to seal the powder gasses behind the projectile/bullet.
Now as to the second vague question, why melt perfectly good shot to cast a round ball? Lots of lead available other places. But yes, in a plastic shell with proper wads it can be done.
As for ? #3, you do realize that tossing about 1¼ ounces of lead at anything over 1200 fps will result in some real fierce recoil? Ed Hubble was using some real heavy rifles, but still had very heavy recoil, simple physics, opposite and equal reaction.
I HAVE loaded the mag tech all brass in 12 gauge and .410. For the 12, I used 10 gauge wads. If you can get them, 11 gauge wads also will work. There's almost no published loads for the brass shells, you're entirely on your own as to how much powder to use.