The lead bullets intended for the 45/70 don't work. These are usually pure lead, and work at low black powder pressures with the smaller bore and different twist than the .450 Marlin.
I am obvously not the number cruncher you are and do not take issue with anything you have written with the exception of what is in bold in the quote above. I have shot the Lyman 457193 cast 40:1, 30:1 and from wheel weights through my Marlin 1895 CB pushed with either 65 gr Goex CTG BP or 54 gr IMR 4350. Bullets sized to .458" and lubed with the same lube for either the BP or smokeless load and a card wad under the bullet base. I also have shot BPCR informal competitions (i.e., not NRA sanctioned) using this bullet in my single shots and have never used "pure" lead for the bullets, nor do I have any personal knowledge of anyone who has or does. Pure lead for ML round balls, yes, but not in a cartridge gun. I suppose you could, but with the velocities you get from a cartridge gun, it probably wouldn't yield any increased benefit (just my opinion).
According to Marlin's website, the 450 Marlin and the 45-70 in the Marlin 1895 have the same twist and number of grooves in their barrels. Granted, the bore diameters and grove depths might vary from one to the other, but they can do that from one rifle to the other in the same caliber, too.
Absolutely, be careful in working up loads. Start at the low end of the suggested manual's load data. Half the fun of shooting is finding that one "sweet" load for your rifle, and that cannot be achieved without a bit of experimentation. But, again, safety is rule one.