If you're using low-pressure loads, your cases should last a LONG time. As pressure goes up, case life goes down. "Full-power" loads appropriate for the T/C and similar guns are hard on cases; "Trapdoor" loads can be quite gentle on the brass.Question...How many reloads would one expect to get out of the 45/70 cartridge??? 3-4??? Before tossing it for good...If that even is close...
Look at some modern pistol rounds for comparison. Some folks have used .45 ACP brass for literally dozens of loadings with no adverse effects; they're being careful not to work the brass too much when reloading (minimal belling before bullet seating), and they're using a "low-pressure" cartridge. The .45 gets the job done with a minimum of fuss and isn't hard on cases at all. I've used range-pickup brass over and over with no problems whatsoever.
Higher-pressure cartridges, like the 10mm or the various magnum revolver rounds operate at much higher levels and exert a tremendous force on the brass, however. Between repeated firings and resizings, the brass can get brittle and/or stretch quite easily. Case life of 3-5 loadings with certain loads is not uncommon.
.45-70 Government can be loaded to either extreme (depending on the rifle, of course). If your primary concern is case life, some light, soft loads at low pressures will allow you to re-use the same cartridges nearly indefinitely. However, if you desire to shoot "grizzly-thumpers," you'll reduce case life significantly.
For primers, use a simple "Large Rifle" primer. Do NOT use magnum primers, unless you know precisely what you are doing; magnum primers ignite powder significantly faster, and raise pressures accordingly. Every loading manual I've read recommends starting over at the minimum loads and working back up if you switch to these primers.I need a powder and primer recommendation...I think my Dad may even have this info (load-wise) written down in his books on this caliber, but thats a needle in a haystack search...
Pistol primers, both "normal" and "magnum," are slightly different dimensionally, as well as being constructed to different standards. If you use these in a rifle cartridge not designed for them, you run risks of punctured primers and wonky pressures.
For powder, I'd recommend anything in the loading manual in the section appropriate to your rifle; while certain powders tend to do better in certain rifle/cartridge combinations, anything that's listed will be acceptable. Stick with the loading manual, and while you might not get "target" accuracy, you'll get acceptable performance for all but the most stringent of tasks.