I removed the cache.img.ext4 file because after flashing the full firmware the phone would not boot past recovery (some issue with a vertification check on a tree, i dunno, didn't make sense to me) and some advice on another thread suggested this would fix. I did have to repackage the entire tar with a different md5, but also not a huge problem, and it still didn't trip the knox counter. It installed fine through ODIN without the cache partition, though I did make sure I did a factory data reset and wiped /cache before booting into the downgraded OS. The device is still functional, no problems.
---------- Post added at 06:28 PM ---------- Previous post was at 06:23 PM ----------
Note that I assume this whole cache.img.ext4 removal would be unnecessary if not trying to downgrade from the OTA PE1/PE2 updates.