Did battery replacement caused any other issues? Did you replace it yourself? I am sort of scared to find my car unusable if I replace the battery myself. I have no issues doing that myself (was doing full car service to my previous Camry before) but this car if fulfilled with electronics and this is the doubt point for me. Yes I replaced it myself (I do all my own maintenance) and it is very straightforward. To protect your electronics always disconnect the negative terminal first and reconnect it last
The symptoms described are exactly what my Aurion did when the battery died, but testing it before buying a replacement wont hurt in case I'm wrong. You will lose any presets in your sound system, but thats all.
There's no need to remove the seat. Just remove the trim each side ( this wont disrurb the airbag), then the deck just lifts up. It might take a firm lift, but it is only held by clips. Removing the headrests makes it easier to get the deck out.
I had a very similar problem when I first bought my Aurion. It drove me nuts trying to figure out what the beep was - til I realised I'd left my phone in the console and It's battery was low. Duhh