Yes to all of that and I forgot to bring in the delay relay. When you go to the Semlax combiner you will loose this hi fan of but getting rid of the .8 volt isolator drop the blower will spin up the same and better than before. I’ve all but deleted the idea of the battery isolator. Specifically the Semlax combiner is so much better and safer it’s a must do. If you have questions on that give me a call but a 1wire alternator and a Semlax ACR-160 is a must. I invite contrasting opinions

Jim, I don't believe they added the high speed relay to prevent the fan from operating on HI when the engine is not running. If they were worried about the fan running with the key ON and engine not running, why didn't they put the fan power though the relay contacts and power the relay coil from the alternator output... that would keep the fan OFF on any fan setting until the engine was started. My understanding is the HI Speed relay is there to power the fan from the higher alternator output voltage available on the input side of the Isolator. This 0.7 to 0.8 volt higher voltage gave the fan a bit more power on the HI position to help overcome the HVAC system air restrictions. Also the close proximity of the fan to the Isolator reduced the wiring distances and therefore voltage drops in the wiring powering the fan on Hi Speed. Moving the power supply point of the Hi-Speed relay to the Battery Positive Terminal removes the higher voltage feed advantage, so your fan will run slower on Hi-Speed. Changing to a one-wire alternator will also remove the higher voltage feed and result in a slower fan on Hi-Speed. Later coaches also had a delay relay in series with the fan power. This allowed the vent flaps to move to their selected position before powering up the fan. The Hi Speed relay can be replaced with a low cost Bosch type 30 or 40 amp 12V relay. The savings can be used to buy some cool beverages for enjoyment after the upgrades.

Maybe the high speed is relay controlled to protect the switch from high current, a lot of cars have problems with that switches, especially in poor europe