After my most recent flush/fill (few weeks ago) there is dex-cool in the system.
*************** Dexcool is not the panacea many would believe. The early versions were pretty poor, actually.
Essentially ALL the good and premium quality antifreezes on the shelves nowadays are totally compatible with Dexcool.
I dont believe your problem came from having Dexcool in the system, and certainly not from having the radiator flushed and replaced with another good or premium brand of antifreeze.
It is still somewhat in question whether Dexcool damaged gaskets, or whether the poor quality of gaskets that GM used were the bigger part of the problem. There were a lot of rumors about Dexcool, not all of them founded in fact.
It is not in doubt that the GM series II 3800 engines had a prone to fail plastic fuel injection plenum, and that the gaskets that GM used were also prone to fail.
In those engines, I believe it is not a matter of whether the plenum will fail, but when it will fail. The gaskets are, IMO, somewhat less of a problem, although I have opened some of those engines with relatively low age and mileage and have seen some ugly gaskets.
You problem might have nothing to do with gaskets or plenum. That is why I suggested a bit of detective work first. A really good mechanic -not necessarily a dealership mechanic - should have the experience and equipment to avoid a lot of trial and error.
I hope you will tell us how this finally plays out.