Try these tests. https://www.gmforum.com/t285281/ I'm leaning towards a bad battery cable.

I'll check it out once I get some daylight. In the meantime, could there be any sensors or similar that would prevent the car from starting if not connected? I think I got all of them, but after how many things I had to take off to get to the LIM gaskets it'* very possible something got missed.

Nothing I can think of that would prevent a turn over. Have you checked the obvious, relay and fuse? You will know a lot more after running those tests. That will either rule out, or confirm.

You didn't unhook a large ground cable from the motor in the process and somehow miss it? I forget where exactly on your year the cable hooks, I think to the bracketry above the AC compressor. Doubt you'd need to pull it to do the LIM thought.

I'd try jumping the starter soleniod to see if it'll turn over that way. That would rule out battery, cables and starter. It would leave us with VATS

Just a wild guess but I figured I'd throw it out there. Any chance the dealer screwed up something and the resistance in the "chip" is wrong on your key? I believe that would cause a no "click" type issue.

That is why I included it in the original post because I don't know what the symptoms of that would be? Can anyone confirm this? They mentioned that if a specific ignition component had been replaced (sorry, can't recall the name) the key wouldn't work since they had to look it up by the VIN. Does anyone know what component this is, and if it had been replaced, would the original key cut still fit?

In the meantime, can anyone provide more details on what is involved in jumping the solenoid? Thanks for all the input.

If you're referring to the ground where the negative battery terminal connects it is on the alternator mounting bracket. I already followed it back and it is securely connected. The starter motor was left alone in this repair.

You can check the resistance on the key by turning the key to on and observing the security light for 20ish seconds. Does it stay on, blink or go out.

SECURITY light is lit solid for about the first 5 seconds after the key is turned to on and then it shuts off. It does not display while attempting to start.

I cleaned the contacts on the cables going to the battery, and the primary ground. I also cleaned the ones going to the solenoid. Still same problem.

I jumped the connections between the ignition wire and the positive terminal on the solenoid and got sparks and rattling, which I'm assuming is a good sign.

Here'* a thought:

With the bad gaskets, lots of oil was leaking everywhere. Yes, the starter is brand new. However, I had to use a lot of engine degreaser to get things cleaned up, which means lots of oil leaking down with it. Is it possible that the starter swallowed enough oily degreaser it needs cleaning or replacing?