I guess I still don't quite understand the point you're making. Both the adapter I used and the JMF adapter have a 3/8 female compression side with a washer inside and a 1/2 MIP side. They both look the same. Once that is installed on top of the 3/8 male compression fitting on the stop valve, you are left with a 1/2 MIP thread. The female connection on the tee is where you have the nut that can move freely on the tee. Tightening that on top of the adapter could, I suppose, overtighten the adapter if you don't put a wrench on the adapter when you tighten the nut on the tee. But once they're connected together you can swivel the tee, so there is no pressure on the adapter at that point.
Sorry if I am not making myself clear or if I am not understanding what you're saying. I am just an old DIY guy, so I may not be using the correct terminology.