Priority Group Activation Failback with HTTP Cookie Insert

That looks OK from a logical perspective.

Rather than checking if the High Priority pool members are available individually, I would just check to see if the number of available pool members is greater than 1, and reselect in that case.

when LB_SELECTED {
  if { ([IP::addr [LB::server addr] equals "low-priority-ip"]) and ([active_members pool_name] > 1) } {
    LB::reselect
  }
}

Give that a test.