if the crossover is still intact, pressure would equalize between the two banks, exhaust would follow the path of least resistance, meaning part of the pass side would flow back towards the cutout.
I do not understand why someone would want lower boost for economy purposes... a diesel does not need to add fuel to boost. If you are at X fuel rate, the more air you pump in the cylinder the more efficient that fuel is going to combust.
you need to use the factory type, the air flow (hot or cold) cools the resistor for the fan speed, one of the knob type (dimmer switch) would overheat and fail or burn.
check your temp sender, 120-130 degrees is not nearly enough to supply what I would call "good heat". If you have 120 degrees and put the blower on high it will over cool the heater core and become cold again.
you can pull whatever weight trailer you like as long as you don't go over the registered weight of the power unit (you can register it for more weight than the factory equipped it for)