Will the monster power fool me into thinking it sounds better?
I have a built-in prejudice that outboard power supplies are not audible. So I could not tell any difference with or without my Monster Conditioner, I used it mostly for convenience.
IMO, the Monster is not as much worry as the 240V you are planning to split. I am not an electrician but as a carpenter/ Building contractor, I have wired some additions (Yes it’s legal) And I have only seen the instance of splitting a 240V one tim on a bathroom remodel. . It blew many sparks and a hole in my pliers when I cut, what I thought was a 120v.wire. I suspect there is a good reason WHY this is not standard practice and I would never run the circuit this way. So the answer from my perspective is a yes to question 1 & 2.
#3 IF you had run the wire in conduit you could just add a couple ground wires to make 2 separate circuits. But since you say Romex , I doubt it is in a conduit. I’m not sure if you should run a ground wire outside the romex. Doesn’t sound right to me. The other solution if you have a ground wire + the 4 wires , you can tie onto it in the junction box for the second circuit.
The batter and safer way IMO, is pull it out and start over with 2 separate circuits
I run all hifi off a single phase and have full time motors like HVAC and refrigerator on the other. I would NEVER mix phases with hifi. Just because you installed 4 !egs doesn't be mean you have to use them, unless you are all MC.
Only you can remove the HTS 5100 and decide if there is an audible difference.