I presume .45-70 ammo can still be fired in the .45-90.
They will! You'll lose a little accuracy because of the extra free bore, bt you may not even notice. The 90 is a good compromise IMO.
You'll find a lot more of the older target rifle in the 45/90.
Even at .99 each, I can't say I'm thrilled with the cost, but if they're decent cases, they should last the life of the rifle with modest loads/