Well, for those who care, I went with the BMW Branded Battery from the dealer. My wife picked it up on her way home from work Friday, and I put in in Saturday evening.
I asked them to pull the one with the latest date code, but was pleasantly surprised to find it had a date code of 12/21 on it, so less than a month old. Resting voltage when received was 12.85v. Perfect...
Took apart the hatch area to get to the battery (why on earth doesn't the E70 have some way to hold the hinged cargo floor open, or E53 did?!?!
Disconnected the negative cable, then all the cables from the 'brick' attached to the Positive terminal, in the hopes of being able to get the whole assembly off to the side, but I couldn't for the life of me disconnect this small two wire connector right next to the flap that opens to access the positive terminal screw (I have more trouble getting electrical connectors apart on this beast than anything else I have ever worked on).
So I ended up having to very carefully wiggled the 70 lb battery out, and the new one into place. But everything went back together without issue.
Resting voltage on the original Battery was 12.35V (done)..
Registered the new battery with the Foxwell, closed it up and left it alone till the next morning.
No issues, other than my numb fingers (it was 21 degrees out)..
The next morning my wife left before I could check on things, and she texted me photos of the DSC and xDrive malfunction warning on iDrive, and to let me know the clock needed to be reset.
I called her to have her perform the steering angle sensor reset (all the way left, all the way right, back to center, turn off car, restart), but she said it had cleared on its own after driving it.
So total damage was really only $201(plus tax) once I return the old battery and get my $20 core back. Still can't believe buying a battery from the dealer turned out to be the most cost effective option
Photos of the new battery, with the new part number visible, if anyone is interested..