So this is your Diesel set: https://mthtrains.com/30-2312-1
I'm not an expert regarding the electronics. Personally, I thought the B-unit only contained a tether to pass power for the headlight through to the other A unit. So therefore, the two A-units could be connected back-to-back and the B-unit's absence would not affect operation.
What about the 4-6-0? Is it the "old-timey" one, or the train set loco? No Proto-2 loco will move very fast in conventional mode without at least 10 volts on the rails. That's normal. The old-time 4-6-0 is geared pretty low and they aren't fast runners; that's a good thing! How about some pics?