Myth No.2: I want to install a backflow valve in case I get a sewer backup in my house. This will save my finished basement?

Fact. - backflow valves or back flow preventers are not to be installed on septic systems because oxygen is required.
Oxygen from the interior plumbing vent on your roof, commonly known as the "stink pipe", needs to make it's way into the tank to relieve methane and hydrogen sulfide gases. These gases must escape before they cause bigger problems.

Oxygen is required to aid in breaking down the solids in the tank. Your septic tank is always creating heat and gases and is essentially a glorified underground compost.