coolant leak. in shop system holds 20 psi, and cant make it drip, no white smoke, Oil level holds... I fill up the rad 2" every few days and it leaks when it cools down...Sometimes

Have they tried pressurizing it with the engine running? I had a small block that would loose coolant but it would not drip, pressurized it and started the engine and bingo I found the leak at the water pump.

It was pressurized hot and cold, no leak and throughout the system the shop says there are not even any stains for coolant leaking down something

The system does have gunk in it from its previous owner who put in coolant leak stop, and its caked on in a lot of different places And the top hose does collapse when the system is cold

Stop leak is not a good sign, problem could be in the intake manifold seals, cracked head outside of the combustion chambers, cracked block, casing flaw in the block, not going to be an easy one to find.

A block test would confirm or deny this idea though?

A block test will confirm or deny the presence of compression gasses in the cooling system, but if the crack or leak is say in the head around a valve guide away from the gasses then it won't show, If the intake is seeping coolant into combustion chambers it won't pick that up either.

What are my options then...

The first thing to do is get the crud out of the cooling system, and it's not going to be easy, it maybe masking the real problem. Or just start tearing it down and checking everything as you go.

Tauruses can leak at the heater and during certain conditions, spray coolant in a fine mist hardly detectable. Leaving the pressure tester attached for a day and overnight has proven these cars to leak in this manner at dealers when they are still covered by warranty. I would point out that timing covers can leak at the lower corner, waterpunps can be tempermental, and 10 tiny leaks add up to one big one. The heater hose ones require being pressurized just below the specified value on the cap. do not exceed the cap's value. give it time, and check and pump up the tester every few hours or more often as necessary. The eventual result can sometimes be from the floorboard under the car, but not in the interior. The engine issues take grat patience to find as well, too bad someone has already become impatient and tried to cover up the cause with stop leak. You could raise the front of the vehicle and fill and burp it, then add it's needed amount to the recovery tank and monitor for a week or so, adding only to the recovery tank. adding a dye to the liquid can speed up the findings upon inspection at a shop with the proper light.

A park avenue has all the same kinds of issues and the parts are quite the same.as to how they fit and respond to their environment. using stop leak of another type can also change the circumstances for the method of Stopping several very small leaks. When I worked for GM, we had some cars with hose attachment leaks. The ones that sucked the upper hose flat were low on coolant. they just had not been lifted when they were filled. Using another brand of stop leak helped because the service manager figured out chemically, what would save time, because the customer had limited funds. Read the labels on what's available to you. pepper works, bars leak works, and leave the solids out on that type. then flush and clean the parts after it has had time to help you. You can invest more time in it when the weather cooperates if you are lacking patience now.