I've never seen a 24-3 or 624 with normal (.429-.430) exit bores. In the OP's gun, I would be loading a .432" cast bullet with pretty soft metal - straight wheelweights, or wheelweights with a dab of tin and maybe a little extra pure lead.

Any reasonable powder should work well and accuracy should be as good as the gun is going to do. I wouldn't pay the slightest attention to the unburned powder thing. It doesn't hurt anything and you are going to clean your gun anyway.

The .003" difference between barrel and cylinder exit bores is more than you'd like but it is not going to be dangerous and/or destroy accuracy. I have been shooting .432" bullets in my 29-2s for years now. As long as you have big enough chambers to handle the loaded rounds (my guns do) there should be no problems.

Replacing the cylinder... good luck!