Wouldn't start; replaced old 12V battery; now won't start again

Last week in a cold snap in the Boston area, my 2007 Prius (150,000 miles) would not start. Despite driving a couple miles earlier in the morning, when I got in the car an hour later, the dash lights started blinking but nothing happened after that. Eventually, nothing lit up. No luck with a Weego portable jump starter, either. Fortunately I was able to get it jump started with a friend's vehicle, giving me enough juice to get home and park in my driveway.

After reading some threads here, I purchased a new 12V battery at Autozone: a Duralast 46B24R-AGM that the Autozone website said would fit my vehicle. However, at the store they said the battery wouldn't fit because that battery is for the very common 2007 with SKS, whereas my car is an apparently rare 2007 without SKS. Indeed, the 46B24R was about 0.75" longer than the battery it replaced (an S34B20R), so I had to remove the bolts for the bracket that the battery normally sits on. In any case, I checked my old battery with a multimeter and it came back at 11.9V, and in my 6 years of owning the car I'd never replaced it, so it seemed I diagnosed the problem.

On starting the first time with the new battery, I got the red triangle of death. But from reading these forums, that seemed somewhat typical after having no 12V for awhile. After a few more minutes, the car powered up and I was able to put it in drive without the RTD. I had to reset the automatic window, and the backup beep I'd previously disabled had returned, but that was it. I did notice that the Power button LED was amber on startup rather than green, but otherwise things seemed ok.

After replacing the battery, my cousin drove the car ~50 miles and said it sounded loud; I'm including that anecdote here in case it means something. I planned on trying to reproduce that issue this weekend, but this morning was cold again, and the car did absolutely nothing when trying to start up - even the remote key fob could not unlock the doors.

Has anyone experienced this before? My fears/suspicions are that it could be an issue with the hybrid battery, which has never been replaced (again, the car has ~150k miles). I don't think the new 12V battery could be the problem, unless having the wrong physical size also means its performance/specifications are also wrong for my non-SKS Prius.