I haven't checked the voltage, but I put a new Optima in and the car starts like it should now. The windows are quicker too.
It's a bizarre coincidence having the battery die right after the starter relay went out. I did manage to fully discharge it earlier in the year, but it seemed fine after being charged up again. It tested fine months later. Maybe it wasn't. I had a LiPo die this year that charged to full voltage but could only provide power for about 15 seconds. The Optima did the same thing, at least, the trickle charger showed full charge but the battery was only good for one or two starts. Finally it dropped to powering accessories but no starts.
I "solved" the thermistor problem by trimming the sides off and zip-tying it to the new battery cable. I think that will be close enough although I suspect the thermistor will read a slightly lower temperature based on the extra thickness of the rubber insulation on the new battery cable, and the insulation doesn't fit as tightly as on the original cable. That may cause the alternator to give a little more charge, but I've read that Optimas like higher voltage, so maybe that's what I want? My guess is it will be o.k.