And as a corollary, LeafSpy will show the actual SOH of the battery, irrespective of SW version on the BMS, correct?
Yes and no. Without the BMS firmware update, the SOH reported will not be honored by Nissan for a battery replacement.
If the BMS firmware is the old version, the only way to know the true capacity is a range test. The firmware update was done to correct a mis-reporting of the SOH by the car.
On my 2017 Leaf, before the firmware update my Leaf reported 92% SOH after 4-5 months. I'm now at 91% SOH after 5+ years with the firmware update. I believe the latter reading since I routinely go 100 miles on a charge.
Bottom line, if the firmware update has been done AND the SOH is low, you have a very good chance of getting a warranty replacement battery. If the firmware update has NOT been done, you will need to do a range test to find the capacity of the battery to either calculate the odds of getting a warranty battery or decide if the SOH is good enough for the price. For a 30kWh battery in normal conditions, I'd say 1 mile/% SOC at 50 mph on flat ground is a sort of base line. Faster speeds, using heat, head winds, hills, stop and go traffic will all affect the range so take those into account. If you can do 20+ miles on a flat highway without stopping and measure the SOC at the beginning and end you should be able to get a rough idea of the SOH of the battery.