Problems With Wiring Harness and No Start Issue

I have a Simplicity Regent, model 2690810. It has a 19.5HP Briggs with 38" deck.

Can someone please tell me what the difference, potentially in wiring/circuits, for harness numbers 1728097SM and 1760255YP? I have a Simplicity Regent, model number 2690810. It has a 31Q677-0123-B1 engine. The listings online, everywhere I looked, would indicate that my machine would have originally had the 1728097SM wiring harness. The machine is a 2009 from what I was told, but the serial number starts with 2014, so that's odd as well. Everywhere I called and also looked, told me that the 1728097SM harness was discontinued, so I ordered the 1760255YP, because it was still available. I have already ran the wiring, and verified connection by connection, plug for plug, replacing the old connection as I connected the connections on the new harness. I printed out the entire, colored wiring diagram, and have been over it front to back and top to bottom, at least 10 times. The only, and I mean only difference that I can see or noticed, is that the original harness had used all 6 spades on the key switch, while the new harness did not have the individual sixth lead, which as far as I can tell, is only an accessory circuit.

I cannot get the machine to start. Including myself, two other technicians, one of which is a master electrician, and the other the service manager with 32 years experience, have 9+ hours in this machine. There's power where/when there shouldn't be, and no power when/where there should be. Key switch is also new, and at the on position there's power, but none at start. Power is randomly hot when depressing parking brake and sitting in seat, when depression of the plunger on seat switch occurs. Engine wiring harnesses are their own thing, and that wasn't changed. The harness should be just based on chassis, and I'd have assumed whether it was using the 19.5HP Briggs or 23HP Kohler, the only difference would be the engine wiring itself. I don't know if I got a harness that was made on a Monday morning or Friday afternoon, but something ain't right.

I am in dire need of help. This is my personal machine, and I am getting reamed a new one for taking up shop space. Please help or advise. Thank you!