Wellfed...., Did you happen to swab (dry OR damp patch) your bbl. after each shot? If not, you may want to try it, as the powder and lube fouling may not be helping your accuracy.
I dry brushed the barrel between shots.
Also, the 2" groups with .445" patched RB's are a bit troubling. A T/C bbl. is capable of better accuracy than that (5 shots in 1" - 1.5" from a rest).
The barrel is an older one and has been shot quite a bit before I got it (judging by pitting etc on the outsize near the muzzle and muzzle). It does well with a Hornady 445 round ball in a tight TC Bore-Butter lubed patch. 2 inches is not too bad for me with iron sights
I must also ask how well that Minie Ball fits your bbl.? If it's too loose, accuracy will likely suffer. (You may need to paper patch it to fit.)
The Minie Bullet is pretty loose going in. I suspect that it is not expanding enough to grip the rifling.
Assuming you don't have a problem with the bbl. or how it fits the stock and tang, I'll try to answer your questions:
1) Pure Pb for Minie balls and Maxi-Balls. I've tried using WW's + 1% Sn and ony succeeded in wasting powder and alloy. OTOH, pure Pb Maxi-'s and REAL's were very accurate (5 touching from a rest) in my T/C .45cal. Hawken.
I used lead that dented easily if I tapped it against concrete. I haven't tried squeezing a Minie ball with my fingers to see if it is soft enough to bend the skirt.
2) This is something you'll need to experiment with. Why not start with 40gr. FFg, maybe 10 shots (swabbing between each shot) v. 50gr. v 60gr. v. 70gr. v. 80gr.? I'm thinking accuracy should improve at 60gr. Btw, after you determine which load is more accurate, you may also want to retest and fill the hollow bases with lube as well.
Thanks, I will try upping the powder charge in increments.
3) The 1:48 twist will stabilize a 240gr. Maxi-Ball, but it may be a tad slow for the 255gr. Minie ball. A bit more speed may help.
Hope this helps!