Single relay!? That would seem to obviate the safety factor that the series switch modification gives. Well, at least in part. I would be horrified to learn there was truly only one point of cut off to that vital flying surface motor.
It wouldn't surprise me to learn it stayed on with a delayed opening, to avoid stop-starts as different inputs to the HStab came in quick succession.
The last relay I had fail was on a Mercedes. It left the secondary air fan on, which wrecked it as it was not continuously rated. When I finally found the darn thing the welded points took a lot of parting. In the car case the fan runs at start-up and shut down. So regular use. On the Boeing, I don't know what current that isolation relay would have to switch during (I presume) pre-flight checks. There was talk in this thread of the motor running - and the clutches choosing the driving logic. I have never found out just how long the H-Stab motor runs for during normal ops.