Tried several more times with R70: deleted sensor, added again, unplugged Peanut Plug, moved it immediately next to the Almond+ (2 inches), etc. No success. Guess I'll wait for R73 and if it doesn't work then I guess I'll try contacting support?
EDIT: I tried downgrading to R69cp, since I'd had some accidental success updating the firmware on the PP using that. This time it said "failed" but the light on the PP stayed solid red. I left everything untouched, and after about 5 minutes the "Update failed" changed to "Update 1% compete"...and it eventually finished. I THINK the same may have worked with R70 (I seem to recall that ONCE the light on the PP stayed solid red for a while), but I got impatient. Definitely not user friendly. Back on R72 now and power monitoring works.