Do you have any dash lamps that normally light up when the key is turned to the RUN position before you turn the key to the START position? If so and none of those lamps are coming on now.......your key switch may have gone defective, or the wiring connector on the back of the switch may have become loose.
The absence of the fuel pump operation may also be an indicator of a problem with the key switch, but not proof of a key switch issue.
Be cautious in using terms like "Everything under the hood appears to be fine electrically" unless you are checking things with a 12 volt test light or a DC volt meter. The naked eye is a poor judge of things electric except for identifying burnt or broken wires.
There may also be a relay involved in supplying 12 volt power to the primary circuits on this machine. Failure of a relay, or a defective control circuit for the relay.......can mimic other types of component problems.
You are going to have to begin checking the circuits using a test light or a volt meter, beginning at the fuse and working next to the key switch for battery voltage. Hopefully it will be nothing worse than a defective key switch or the connector on the back of it.

I also failed to mention that the red and green LED's show no power on the printed circuit board. To answer your question about lights on the dash, there are none. Would this be indicative to no power going to the circuit board at all when the key switch is on?

According to my schematic, this has a fusible link.

Look for the RED wire between the + terminal on the starter solenoid going to the key switch B terminal.
There should be a section that is smaller diameter.

Any fuses are on the "other side" of the key switch.

Mownie offers good advice about checking the connector.

What's the serial#?
There's different schematics before/after 70000.
Diesel or gas?

I dont know the wiring but I know there are times when things looks right, but you forgot something basic. Like my brother who left his car on the interstate for a couple of days because it would not start. When the tow truck came I realized the car was in drive and is designed not to start.

Or the spring I took the battery that I had on the charger in the garage to the shed and put it on the tractor. It would not start,after running around for an hour, and a "few" curse words I realized I had left the mower blade engaged, and it was designed not to start with the blade engaged.

Just a comment.

Serial number is SKA617U1G2RA. This is Gas.

OK, I pulled out the key switch and used my meter to test for voltage. There is none. I do have power to the one of the two fuses on the circuit board. It is the top fuse that provides power to the switches on the machine. The bottom fuse provides voltage for the lights as I understand. There is no voltage for the lights fuse present.

I could send you a link to the tech manual, but you have your email disabled.
I can't post the link here.

If you still have that link id love to have it. I am currently having the same problem.