Unless you don't need one. 60k+ miles on my Challenger with that brake set up, and with 1.12" torsion bars up front, 275's all the way around and even the 11x2.5" drum brakes out back it would lock the front's slightly before the rears. No adjustable prop valve can change that (they can only reduce the rear pressure), none needed. You only need one if you need to reduce the pressure to the rear brakes. And if you're not constantly swapping suspensions or tires, you only need to set it up once.
Exactly. There are a lot of factors that determine the brake bias. Tire size, brake size, wheel cylinder and caliper bore size, front/rear weight balance, spring rates, etc. There are a few different rear wheel cylinder bores out there for these cars, but if I needed to back the rears off I would be more apt to install an adjustable prop valve, just makes it easier to tune.
Yes the power brakes would make a difference, the linkage for the power brakes actually has an effect on the pedal ratio. A and E bodies didn't even run the same master cylinder bores with power brakes because of that, the A's were actually backward, they ran a 15/16" master with power and 1 1/32" with manual. E's were the other way around, and it sounds like the Cordoba's used the 1-1/32" with power as well.
As for running the power brake master with manual brakes, you will have to check to see if they have the provisions for the manual rod and retainer, they probably don't. The aluminum ones don't, that's one of the things Cass does to modify the later aluminum master cylinders for use with manual brakes.