I'm attaching the service bulletin pertaining to the two valves. It'll show you how to install them. They're intended to, over time, bleed whatever air is left in the system. My ex had a 2007 base model that had this problem for years before we started dating. Her various mechanics changed the thermostat more than once, changed the radiator, performed many flushes, and the final suggestion before we met was "you just need a new engine." I did some researched based mostly on what I read here and had it fixed within a week. She kept watching the temperature for another year. That old habit was hard to break, and a major stressor.
Your pressure fill won't work. The problem is that there are areas where air is trapped above the waterline (the height of the overflow bottle) with no way out. Adding pressure just tries to compress that air in those locations. The air may compress a little but once your pressure is removed it just expands back. Vacuum removes all of it. Then you put coolant into the opening and allow atmospheric pressure to push it into those areas, since there's nothing there to oppose it now.
I bought vacuum tool off of Amazon that works with an air compressor (venturi effect). After installing the two valves, prepare your coolant mixture in a big jug. The tool comes with a valved nipple for a hose that you put into the bottles. After achieving vacuum, you close one valve and open the valve with the tube and the fluid is blown into the system through the tool. With a little practice you can vacuum well enough to put in the full compliment of coolant (the specified system volume), but it's not critical. Even getting most in will generally block trouble areas with coolant and, being vacuum pockets rather than air, they'll eventually fill. Be sure you use the water pump drain plug to drain the block in addition to the radiator drain. Otherwise, you may have air trapped behind residual coolant that the vacuum won't remove.
Regarding bypassing the heater core, I'm attaching a line drawing of the thermostat housing showing the two lines. They're so close together I would think you could just cap each one off. No need to loop a hose between them. The return nipple is the one that's directly behind the thermostat. So, you can see why using cabin heat ends up putting cooler coolant onto the thermostat back, causing it to close up (and engine temp to rise).
ha ha... thanks for complimenting my girlfriend. She's a lot of fun and into costuming. She owned a Solstice years ago and now wants my Sky.
BTW, my preference is to run 100% distilled water with a bottle of Redline Watter Wetter for the additive package. I get lightening fast warmups which gives me quick oil warmups via the oil cooler, and that all results in reduced wear and improved drivability. Also, it tends to tie the oil temp more directly to the coolant temp, aka, oil runs at an ideal 210F rather than 230+. No freeze protection so it stayed in my garage during the great Texas freeze. Our other cars, I run 25% coolant, which just barely covered the recent freeze. They're all garaged but always the possibility of getting trapped outside on a trip.
