I had the same issue with not enough pressure to keep the backflow "plunger" sealed using the connector closer to the house . . . but I had 2 quick attach connectors on ball valves on there already and I just flipped one 180 degrees so the shutoff was still between the breaker and the manifold but the hose connector was on the other side of the shutoff. I just shut that off to blow out the system and it eliminates needing to have the pressure to hold the plunger in the sealed position. All the air just goes right down the line to the sprinklers.
Just blew my system out about an hour ago. Hit each zone once to just clear things out ahead of the cold. Planning on turning the water back on again probably Sunday, since the warm weather looks like it will make a return for at least a couple weeks. Want to be able to water just a bit longer, so I'll have to do a more thorough blowout in a few weeks.