Well, this seems to be the case of the 'vq35HR' syndrome (High Rate).. Any place you go and as soon as the 'mechanic' sees an HR engine, they ramp up their rate by 400%..
It's an easy to work on engine and to blow a head gasket without any forced induction is slimmer than you winning the local lottery jackpot.
Freon usually leaks by the connections next to the driver side headlamp but like mentioned above, you would see the juicy and oily spot where is leaks.
First step, wash and degrease engine bay, then you can pump the cheapest 134a can of freon into it and see where it leaks. Or get a o-ring kit from amazon and replace all o-rings on all connections, get the vacuum pump from amazon for under 100, watch few youtube videos and do it yourself. That way, you will save thousands and learn something new..

 

Right this is what I'm trying to understand. The shop I use is very good and only works on Infiniti & Lexus but not sure if the best qualified tech looked at it or offered his advice. Can a leaking head gasket or cracked head truly blow air into the coolant with no other evidence of a head gasket leak like fluids mixing, running rough, etc.? Pressure test passed at two other mechanics in the fall.

I really don't care about the AC right now and can live without it for the summer if needed. I have covered parking at home and work and most trips are 5 miles or less. It's stupid to even think about fixing AC if the engine may be getting worse.

I won't be doing any major work myself but may teach myself how to flush/fill coolant in case that helps get the air bubbles out.

 

Well, this is the 'mechanical business' part and different shops charge different prices. Since you have 2 other tests passed then 2 out of 3 is better than 1 out of 2.
Blown head gasket can mix coolant with oil but not all the time. Mostly, coolant will be overheated and system will be over pressurized and start vomiting excess liquids..
Basically, the hot exhaust gasses are entering the cooling chamber so the pressure must exit somewhere, like a blow off valve on turbo induction..

First, i would replace the thermostat, radiator cap and buy this funnel to 'burp' this engine from all the air: You can find plenty of videos on 'how to' on uTube.
-Thermostat if old and lazy, can slow down coolant flow therefore overheating is possible.
-Radiator cap if old and leaking, it will reduce boiling point of coolant and possibly allow air to enter the system.
-Burp the engine like burping a newborn after feeding; If you don't, then the newborn will throw up all the air it sucked in..

At the same time, inspect all coolant hoses and all points to see if anyone of them has a pinhole in it and is leaking/sucking in air into the system. I had that issue with those stupid metal throttle body heater tubes behind the intake manifold where one somehow developed a pinhole and kept draining my coolant.
Second, i would wash the radiator with purple degreaser and then air blow it. Dirt gets trapped in those fins and can slow down heat exchange especially in hot climates.
As for the AC, it's more complicated but once you learn it, it's very easy to maintain.

 

thanks i'm willing to throw some minor parts at it. i've already replaced radiator cap and didn't make much difference. also going to try replacing this on my own since the original is at risk of failing, though supposedly this one isn't as prone to failure.

https://www.z1motorsports.com/z1-pro...e-p-11190.html

is it better to replace thermostat from the top or the bottom? i'll probably get a friend to help with that. makes sense to do it at same time as bleeder port/connector.

 

You can use any brand T bleeder for this car, they all do the same thing. The fill 'burp' funnel works as the final air bleeder, the T bleeder is basically gonna tell you "i'm full of coolant" during refill and then you close that and use the funnel. Also, there are few different pressure rated rad caps like 1.3 bar (18 psi), 0.9 bar (13 psi), i've tried both and stuck with the with no issues in SE Florida weather.

Since you are replacing the t-stat, buy this: from there or locally and flush out your system using a garden hose before putting new coolant in. It's concentrated so buy a gallon of distilled water to mix if needed. Remove bottom rad hose and jam the hose in the top rad fill port and let it flow until clean. Backwash the heater core by jamming hte garden hose into the In port and the Out port. Better way would be to remove the radiator and backwash it upside down (mine had black gummy bears inside) since you don't have AC to worry about and you'd have much more room to operate on that t-stat from the top. Make sure to buy OEM t-stat and gasket, i've had issues with so called OEM from Amazon and eBay where they had different neck designs.