I read somewhere that NK1 cannot be downgraded, but what I am gathering from this thread is that it is possible to just apply OB6 or OF1 using Odin then use the Arabic Tools to root? If true, that would make my weekend.
That is
possible, yes. As you see from not only this thread but also the two Yemen threads, some people succeed at getting root (on OB6 or OF1) with those tools and some people don't The rooting method seems to be sort of fragile at best.
Before you go racing off though, remember something else: CM13 needs a custom kernel - so even if you can root, you would still need to unlock the bootloader afterwards in order to be able to boot a custom kernel.
An additioinal prerequisite of the unlock method is that the CID value of the eMMC flash memory chip on the phone needs to start with 0x15 (indicating manufacture by Samsung)
==> You don't need to be rooted to check the CID value - just download the "
eMMC Brickbug (Vinagre)" app to find out what your CID value is before you do anything else. If you are 0x11, you aren't going to get CM13 running whether you can root or not. Only a little value in rooting if you can't unlock the bootloader IMO.
Is the best upgrade route (recognizing that best is subjective) to CM13 going to be using Odin to apply OB6 or OF1, root, unlock bootloader and apply CM13?
Yes, more or less. "best" here really only means "the only way that is available without developing some brand new rooting method".
The incrementing of the firmware anti-rollback occurs at certain
trip points, which appear to be the releases: MJ7, NC4, and NK1.*
You can not revert backwards prior to the nearest previous (to your currently installed release) of any of these three. Of course, you can always flash forward - possibly going beyond and establishing a new trip point.
(* Because of a lack of reporting, It is possible that NJ6 is the actual trip point, meaning that someone on NK1 could Odin flash back to NJ6 at the earliest. Unfortunately, after I
put together all those recovery-flashable Stock ROMs, I installed NJ6 and tried using Towelroot v3 - and no joy. It works on MJ7 through NC4, but not beyond that. So while that possibility (of Odin back-flashing to NJ6) still remains, there doesn't seem to be much of a reason to attempt it.)
For your NK1 unit, that means you can flash forward or backward at will (using Odin) among this group of releases:
NK1 OB6 OF1 PL1
At this point in time - since that device is on NK1, and there are no trip points above NK1 (yet as of 4/2/2017) you can Odin flash whatever you want within the above group.
good luck
PS Note something else that falls out of the above analysis: If for whatever reason you can't root or you can't unlock (CID starting with 0x11), it is probably the right thing to do to flash the PL1 firmware. Might as well as it has the most recent security patches available if the device is to continue to be in use as a daily driver. You can always flash back to NK1 - right where you are now - in the event of some new development or rooting opportunity.