In regard to the PG antifreeze, this article from West Marine has some good info about the different types available:
Most of the cheap stuff is PG diluted with alcohol which explains the inconsistent refractometer readings. Also remember that the antifreeze will mix with residual water in the system further reducing its freeze point. The alcohol can also evaporate from the mix further complicating things. You can minimize this concern by using a higher quality (more expensive) pure PG product designed for engine winterizing.
Over decades of boat and RV winterizing in New England winters I've never had a problem with the cheap stuff in my water system, but I wouldn't use it in the raw water system of a marine engine.
Agree with a previous poster that it is the pressure buildup caused by water freeze expansion that can be most destructive, there is a study that addresses this:
So after winterizing the fresh water plumbing it is probably a good idea to open a faucet to prevent the possibility of any pressure buildup.
In regard to filling the fuel tank for the winter, that minimizes the air in the tank which minimizes condensation and reduces water in the fuel.