Ok so I pulled the rear pads today and lubed all the contact points with CRC orange paste
Left the e-brake adjustment alone for now in order to determine if the CRC did the trick. After driving up to temp and using the brakes a similar amount to previous squeak inducing levels I can happily say the noise is GONE. I hit all the obvious parts of the outer surface of the pad as well as the clip contact points that hold the inner pad to the caliper piston with the CRC. There's absolutely no squeaking at any speed and the job took all of 45 minutes to do.
I'll report back after a few days, but the initial result is problem solved.
1. There's a DIY for the rear pads that shows the removal of the caliper through the hex bolts on the rear side of the caliper. This is unnecessary, instead remove the dust caps that cover the 7mm allen studs and remove those studs completely. This will allow you to remove just the caliper instead of the entire mounting assembly.
2. If you're not replacing the pads with new ones then you won't need to worry about having a c-clamp or similar device to push the piston back into the caliper. They will come out a bit once you remove the caliper from the rotor, but you can simply push them back in by pushing against the inner pad and refit the caliper once you've lubed all the surfaces. I did this without removing the reservoir cap as well since I only had to move the piston a 1/4 inch to refit the same pads.