I have had a number of 127 SW models (newer and older). The newer ones have longer dovetail mounts. I have never had one flop around. the older ones use a specific dovetail-type mount that is held on by 2 screws. I also have a 6" which is quite a bit heavier and again, no issues with the dovetail.
Regarding the Schott glass, is it meaningless? Yes and No. Schott glass is the actual name of the type of glass that is used and Corning swiped it and called it Pyrex. This, given to me directly from SW when I wrote to ask them about Schott glass (as I had 20 127 models, one with and one without).
I also have a Nexstar 6, 8, Meade 10SCT, and recently picked up an 6RC and 152 achromat. I still have to test them all against each other (when weather warms up a bit) but I am always pleased with the 6 SW Mak.
My wife and I were looking at Mars when it was closest a few months back and she said "keep this, it is the clearest I have looked through (the 127 Sw Mak.)
As long as the collimation is good, you shouldn't be displeased with their product unless you have the odd lemon and that can happen with almost any manufacturer.