You might have a poor connection at the safety disconnect (BST I think it's called) on the positive terminal or maybe the firewall terminal. Do some more testing with your multimeter and try to do some with a load on or maybe a helper trying to crank. Another good example of why everybody should really have 2 working keys!

I did open the cap for the BST and double checked the tightness for that bolt also. I figured I'd check everything for the battery so I won't have to do it again. Could that've been the issue?

For the firewall terminal - you mean the bolt next to the strut tower right?

I'm definitely getting another key once I get this damn thing running again.

Yes, that could be but you need to some more checking with a multimeter. Yes, the jump terminal, test there but also would be a good idea to take that apart and clean it up anyway if your testing skills are lacking. We've had a few people get bamboozled by both of those.

I managed to find that the under hood positive terminal bolt was loose. Loose enough to where the bolt and terminal ring was barely flush with the firewall. So I tightened that up. I took the battery out and recharged it overnight again just to cover my bases - although it read 12.8v beforehand (my OC is annoying sometimes). Ensured all connections were tight back there.

First attempt it cranked but no start, second attempt it started right up. Now I'm getting consistent 13.7v - 13.9v on the OBC Menu 9 during idle/load with IHKA/headlights on. I guess I was bamboozled also. I feel like an idiot. Hopefully I won't have any problems from now on.

I also noticed there's a discrepancy between voltages read directly from the battery posts vs the hidden menu. Now I just need to get another key. Thanks for all your help.

I had that something like that happen on an e60. The bolt on the back of the alternator was loose. I did not figure that out though, a shop did. lol props to you