So your car does not run with the coolant under pressure? How does yours not boil? The reason that you shouldn't remove the cap when hot is that removing the pressure allows it to boil.

The way coolant recovery works is: if the system goes above 15PSI the pressure cap releases coolant. As the coolant cools down the pressure goes down and draws the coolant from the tank to fill the system.

Exhaust gas in the system does not allow the coolant volume back in, so the system goes to vacuum collapsing the hose. Pressure drops, not volume. Or, some such science.

In a perfect system coolant will move back and forth between the engine and the tank all the time.

BTW; 2009 SS has a coolant recovery tank, not an expansion tank, or a surge tank.

Your correct. I looked at it this morning and the rad cap has to crack open at 15 psi in order for coolant to flow to the tank.

In other news.

No collapsed radiator hoses this morning, And since the car has been driven a hundred miles or so since the plug change I checked plugs 1 and 2 today. The electrodes were comparable in appearance, however the threads on plug one were coated in an oily substance almost up to the top of the threads. It did not smell like coolant at all and there's no signs of rust. I used a flashlight to look into the combustion chamber and it was bone dry from what I could see. No evidence of a valve cover gasket leak or oil pooling in the plug hole so it must be coming from the cylinder.

Since the threads obviously aren't sealing well on plug 1 I'm wondering if this is why compression was low when I tested it. The tester only has 3/8" or so of threads while the plug has 3/4-1" of threads which would exacerbate the compression reading issue. This would explain why I have good vacuum at idle and no apparent ill effects from a head gasket issue. Thoughts?

Get in and drive. Test it next week

Check your oil after an overnight cooldown. Mine would suck coolant in overnight (via the oil cooler). When warm the oil apparently has enough pressure to prevent this, but when it cools... in came the coolant.

I tossed a rod through the oil pan about 3 days after finding this. It also jumped the timing chain at or around the same time at 143k miles. I hope your luck is much better...

A quick update on the situation. I've not been able to make time to run a compression test, but I've driven 700+ miles since the incident two weeks ago and the car isn't showing any signs of head gasket failure aside from the low compression test in cylinder one. The oil and coolant aren't contaminated with each other, I'm not using coolant, no change has been observed in gas mileage, no white smoke out the exhaust, etc...I also ran another compression test with oil on the threads of the adapter and got 138 psi average over three tests. This suggests to me that the rust on the old plug thread may have affected my first test result since I didn't clean the spark plug hole after removing it. The new plug I installed is also in good condition and hasn't rusted like the old one, so that's good news.

I still want to do the leak down test, I just need to make time. But so far it looks like the damage was minimal if any occurred.

FYI, Orange ring around the outside of a plug on the insulator is commonly referred to as a CORONA STAIN. Has nothing to do with steam or rust.. Google - plugs corona stain

You're talking about something different than what this thread(and Donbrew) were discussing.
The "Corona Stain" as you call it is the discoloration of the ceramic insulator. What Don and the OP were talking about was actual rust on the plug(s).
Since the OP's pictures seem to now be gone, I guess you didn't get to see what he found when he pulled his plugs.

Hoping the original op gets to google corona stain and see what it is..