Max-E-Therm Relays

I currently have a 333k BTU Max-E-Therm on a 13,000 gal pool. I'd like to use the relay connections for pool and spa on the heater as a way to remotely control a the heater, and a high and low setting. (Activating pool = low, spa = high.) There is very little info in the manual on this, but from the wiring diagram/description, it looks like I can attached two relays to the board to activate the two modes. The notes do say to use a relays with logic level switching and to use at least two relays per heater. I'm actually planning on using a single unit that has two outputs, thus would be able to handle an either/or requirement (which is what I'm assuming is the requirement there.)

Just wondering if 1) this makes sense and should work, 2) if setting both relays to "off" shuts down the heater or if I have to additionally attach a relay to the fireman disconnect, and lastly 3) if there a reason that the local controls must be disabled, or if both the relay and panel can be active (the manual states to move the jumper to disable the local panel when using the relays.)

If anyone has any experience with this it would be appreciated!