Jerry, take a step back and ponder need before doing something you’re likely to regret. The .45-70 is fully capable of killing anything in NA and using a wide variety of bullet weights/loads. Making it a -90 will solve nothing and move you further from versatility.
1. Hard cast is of no value re: reduced loads. Softer alloys are the path, especially with BP.
2. DO NOT load light charges of BP for reduced loads unless you fill the void with wad material.
3. 2FG will work fine.
4. Look around at load data, you’ll find a number of recipes that will work. Maybe poke around and find some round balls and go squirrel hunting?
Example from Lyman cast bullet load manual:
293 gr Lyman 457191, 28.5 gr IMR4198 = 1327 FPS.
Same bullet with 12 gr Unique runs 1200 FPS. Both require Dacron wad
Essentially a .44 Mag load, and the list goes on.
The other extreme for your gun could whack an ele....