FFFG is the powder. Grain and Gram are both, in this instance, units of mass.
you can put anywhere from 12-15 GRAINS of FFFG powder in the gun, or 0.8-1.0 GRAMS of FFFG powder in the gun. They just did the conversions for you, to be nice.
There are 15.43 imperial grains to the metric gram.
Not if you want a second shot, no

. 'Cabela's little instruction pamphlet' is a generic document. Use the one that came specifically with your revolver.
Apparently not, no. That would be about TWICE the manufacturer's reccomended loading. Quite likely, it would be mechanically unsound, ie, 'the frickin' thing would blow up in your hand'.
Like the gun's manufacturer said: use a .454' diameter ball and 12-15 FFFG grain tolerance/0.8-1 FFFG grams tolerance.