... turn the key on, gauges sweep, headlights come on, horn and turn signals will work, but when you hit the starter button, nothing. Headlights do not cut out, and there is absolutely no response. ... I am leaning toward ignition switch again but thought I may be overlooking something ......
It is unlikely the key-switch and especially since you have a headlight-current bypass in play - but also, since the headlights are on, that validates that current is at least flowing through the headlight/starter relay, so that circuit must be complete already.
The first clue is that 'nothing' happens when you press the start button - specifically, the headlights do not go off.
That means that the headlight relay is not changing state:
it requires two signals to enable that relay -
1) the positive signal from the start switch
2) the negative 'enable' signal from the ECU
The positive signal (White/Red wire) is fairly easy to check - measure for 12V with respect to battery negative with the ignition on and the button pressed.
I would suggest unlikely the problem is in the positive signal - the switch only has to supply about 140mA of current, so poor connection is unlikely from pitted contacts such as you might see in bikes which are directly switching the solenoid current (several amps in their case)
Then you have the negative signal, which originates from the ECU.
That signal is an interlock that depends on several inputs - the clutch switch being the most notable, but also the system voltage and the side-stand and neutral switches.
It always requires the clutch to be made and depending on the gear selection/neutral will also determine whether the side-stand is a factor.
Normally I would just point to the clutch switch as the most likely cause here - however the other behaviour (of GPI) would not be affected by that.
The second clue is the behaviour of the GPI - which do you have, the DS or the ATRE?
Regardless, this is going to also be related to the ECU
To where did you connect the Switched Power source for the GPI (whichever one you have)
If you connected to the Park Light circuit, then same status applies as per the headlights being on - that power sourced by same pole on key-switch.
Question - is the Neutral Light on (on the actual OEM instrument) ?
What happens with the GiPro if you select a gear?
Now - it COULD be a problem with the Ignition circuit itself - the fact that the gauges sweep I would tend to discount that - but you should validate that by measuring voltage (while in fault condition)
Check the voltage at Fuse # 5 with the key-switch on and the Run/Kill in the Run position
(set the black probe to the battery negative then leave the fuse in place and just touch the red probe to either of the metal tabs on the top of the fuse - voltage at both tabs indicates the fuse is intact)
The voltage should be approx same as at the battery positive (there may be a slight loss)
You can similarly measure the voltages at fuses 8 & 9, which will again validate or question the integrity of the key-switch circuits.
If that is all good my next suspicion would actually be the ECU Power Relay
As matter of course, you can try swapping this one with the Fan Relay position (at least for test) - don't swap with the pump relay just in case the ECU one is indeed the problem.
I would expect the ECU relay could account for both symptoms you are seeing