Hi,
A well designed and installed drain back system should not need any anti-freeze.
You have probably worked on this some already, but I'd have a look at the whole collector and plumbing system to make sure you have downslopes toward the tank everywhere. Then test the system to make sure it drains back reliably. I think that one way to be certain all the fluid is draining would be to turn the pump off, and let it drain fully on its own, then introduce some compressed air in the return line, and see if it pushes any more fluid out -- if it does, there is fluid being trapped in the system somewhere.
I guess that if you do use antifreeze as a backup, that it would have to be good for your coldest temp, otherwise its still going to freeze?
Is the valve a pressure relief, or a vacuum break? There would really be no reason to have a pressure relief on a drain back system, as it is vented to the atmosphere, and should not be under pressure at all. The vacuum break is used on some drain backs, but if the plumbing is right, it is not needed, and its just another failure point.
I've got two drain backs in MT (very cold) -- never had even a hint of a problem with them.
One problem with introducing anitfreeze into the system is that you have to be careful to check it each year and replace it when necessary.
Gary