Some more data, may or may not be useful, but I'm still trying to figure out how an extender pack will behave with DC fast charging. Here are 2 "standard" 18650 LCO batteries (Samsung ICR18650-26H 2.6AH, technically 2C charge rate allowed, and Samsung ICR18650-22P 2.2AH, 1C charge rate allowed), charged at 2 and 3C rate vs. the Leaf's charge curve:
So far my takeaway from this is even at similar or higher charge rate, the 18650s still have a much slower voltage rise than the Leaf's LMO battery, so if they were to be charged in "CV" mode at the same voltage as the Leaf cell over time, the charge rate may be significantly higher than 2C. I'm trying to put together the equipment to do exactly this, which involves either putting a cell in parallel with a real Leaf cell and charge the combination at around 100A, or charging the 18650 alone with a high current programmable CV power supply.
Mux, I would be very interested to see some data on how much current your batteries are taking during a fast charge session (LeafSpy log for the Leaf battery + external current sensor/clamp meter for the extender pack), since this is still one of the biggest unknowns/challenges in my mind, unless you're using high C-rate capable hybrid batteries.