Yes, I agree with quasi-balancing the outputs…. but I would not bother with trimming unless you are thinking of driving huge long lines. My bsolution would be to run the outputs through a series 100R, and connect the return leg to ground through a 100R and a 100uF cap. This gives all the advantages of balanced working…. you lose a little level but +20 should be enough for all but the most extreme of old US systems.
The buzz from the mains tx is fairly normal… the phantom power on those old desks was a bit primitive.
On the channel boards there was provision for pre or post fader feeds and we made a number of ‘specials’ so it’s quite likely that the arrangement you have is original.
I have had many questions about re-capping and I always advise against it unless there is a real problem with performance. In normal use the capacitors should last for about 30 years unless the operating temperature is very high… as it was with old valve gear; in that case the coupling caps tend to dry out….. Basically, don’t bother unless you have to! 