It's not the Trump Plaza but it'll work!
1) Yes, the 15.5 ohm point is the 'center' of the simulation but since the pot is 25 ohms, you can set it 'exactly' based on the final circuit values. The value of R2 determines where the relay will turn on and thus at what temperature the circuit fires.
2) Rf is called a "feedback' resistor giving the circuit some positive feedback to help the transistor switch. It does nothing to the MB CTS values.
3) The SPAL fan does require lots of amps so the circuit as shown would need some 'beefing' to withstand the power dissipation. For example, with the fan 'half-on' the power dissipation in the transistor would be 6 - 7Volts x 22 amps = 143 watts
!!! That would reguire a substanial heatsink and a TO-3 case style transistor. What comes to mind is the blower regulator module on my car. See the pic in this thread:
A/C blower's output is low . . .
BTW, did I use the right values for the MB CTS that you are considering?
I really don't want to build this circuit but I'll look around for some 'device' that you can easily modify for your needs. There's got to be someting that's just right.
PS: On second thought, what's wrong with using that blower module??? Other than it's fairly expensive ($170)??? I think it could be used!!!!!