No, a pressure reducing valve is not a back flow preventer, and neither one can be considered a check valve. These are all different beasts that serve different purposes.
(See PS about mixing hydronic water with potable water below:)
My understanding is that there's no actual "check valve" per se built into the pressure reducing valve, but the low pressure of the hot water heating system (typically about 12 to 16 psig) prevents back flow of water into the water supply piping (typically 40 to 80 psig). That is, water won't flow against a pressure gradient any more than it will flow uphill.
Where I live, in Winnipeg, Manitoba, there's a city by-law requiring a "back flow preventer" on all commercial boilers (which includes hot water heating boilers in apartment blocks). The concern is that if the pressure in the water supply piping drops to 0 psig, or event drops below atmospheric pressure, then the low pressure or partial vaccuum in the supply piping could suck the hydronic water out of the boiler through the pressure reducing valve. A back flow preventer senses that drop in pressure and allows air to be sucked in to the water supply piping instead of the hydronic water. In that respect, it differs from a check valve.
PS: Drinking hydronic water. It's water from the same water supply piping that supplies both your kitchen sink and your boiler with water. So, where's the concern about mixing potable water with hydronic water? I never understood that concern.
In a hot water heating system, the new water that goes into the heating system has both dissolved oxygen and hardness ions in it. With a few days of heating that water, the hardness ions precipitate to form "scale" in the hottest parts of the heating system (the boiler). Also, the dissolved oxygen reacts with the iron in the system to form a black form of iron oxide (Fe3O4, I think) which causes the hydronic water to gradually turn black. And, rust isn't poisonous. It's not as healthy to drink as a V8, but from what I know, it's no more harmful than drinking rusty water from rusted cast iron piping before it clears.
Once the water in the heating system is both oxygen depleted and ionically dead, no further changes occur to it except that it repeatedly gets hot in the boiler and cools down in the radiators.
So, unless someone is using corrosion inhibitors in their hydronic water, I really can't see why drinking hydronic water would be unhealthy.
And, in fact, I know a girl who spent time in Russia prior to the Soviet Union collapsing in the early 1990's, and it was common for people living in apartments there to open the air vents on their heating radiators to get hot water out of those radiators for cooking and cleaning and making tea. The reason why was because electricity was unreliable, and so if you had an electric stove, you couldn't count on there to be the electricity needed to heat the water in your tea kettle
24 hrs a day like we've come to expect here in the West. So, Moskovites would use the water out of the building's heating system (which was heated with gas or heating oil) to make tea and for cooking supper. And, the building maintenance people would intentionally not use corrosion inhibitors in the heating systems because they knew people were drinking that water and using it for cooking.
I'm not encouraging people to sample the water from their heating systems for taste. I'm just disagreeing with the premise in the original post that "I realize that mixing of the forced hot water heating system water, and
the home water (washing and consumption) supply is a real no-no, as it should be." I just don't see where the no-no comes from if nothing is added to the water after it's entered the heating system.