GRD, good questions.
The board is designed so that the phono RCA shields and chassis ground come from the same point. That is, one piece of solid chassis forms a common large single point ground. From there, a few ground wires to PCB along one side connects to ground plane. Currents on the ground plane are arranged for minimal interference between circuits. It really doesn't matter too much where the IEC (power cord) earth gets connected, as long as it mates to chassis somewhere and can handle many amps. It's mostly for safety reasons.
So if you build per instructions, there should be virtually no hum. If you want to move the RCA jacks to another part of the chassis, then it becomes appropriate to isolate the return shields. It also helps to run coaxial wire. These are very sensitive low level signals, and of course, maximum gain is at power line frequencies.
One thing I may not have mentioned in the manual, metal standoffs will also connect ground to chassis via the two holes near the i/o jacks. Normally this is ok. But with jacks moved you might get some looping.
You are fine with the way you built it. I prefer to run a ground wire paired with each signal wire.
jh
