Looks doable to me, wiring the pot up as a rheostat (as opposed to a voltage divider).
No need for the buff'n'blend.
Use a DMM and see if the PCB's PAD-1 of the Switch goes anywhere at all (it doesn't on the schematic in the build doc, but best make sure on the actual PCB).
If not, then you can jumper the pots LUG-1 to LUG-2. Not strictly necessary but provides some insurance if the pot fails.
Or, you may find you prefer the signal from PCB's PAD-3 to go into LUG-1 of the pot, jumpering lugs 2&3 together (which should make the pot work in reverse).
Depends on which way you want the FULL-BLEND to be, fully CW or fully CCW.
What value of pot to use is beyond my ken; somebody more knowledgable than myself would know what to use. Try B2k, B5k, B10k, B20k... keep moving up until you get the right feel that you want.