Pyrodex contains a HUGE proportion of it's total mass of a compound called "perchlorate". This is the oxygen producing compound in that propellant and the fouling generated by it, is very corrosive with the addition of ANY moisture at all.
I use 3F in the .45 and .32. I find 2F more accurate in the .50 and larger calibers, however I developed extremely accurate loads (just as accurate as 3F) in my .45 & .40 using 2F, just in case I ever ran out of 3F.
No - 2f does not foul any more than 3F - for us, at least - I have NEVER seen this and have been shooting both since 1972.
Because 3F develops more pressures per grain than 2F does, maximum pressure loads for any given bore size when using 3F will be reached with less powder weight, than maximum loads would be in 2F.
When the same maximum pressure is developed using 2F as is developed with 3F, the 2f load will usually be producing not only better or the same accuracy, but in larger calibres, from .50 on up, higher velocity, which means flatter trajectory. Yes - the 2F load contains more grains weight, ie: fewer shots in a pound of powder.