Thank you guys, made some progress. Using v1.9 on another computer, I was able to create the twrp recovery image. However, when I attempt to flash it, I'm getting the SP Flash 6045 error re: missing signature, etc.
This leads back to my original question:
Is is possible to successfully flash an unsigned recovery or boot img on a device that uses boot-sign.img, recovery-sign.img, etc?
Is it possible to re-sign this twrp recovery so that it can be flashed w/ SP Flash?
Is it worth trying to flash it via fastboot?