Aright so you haven't heard of any issues that make the battery behave like this
Yes I have, this happens all the time with third party batteries. you get what you pay for. Do yourself a favor and just return it. Then take it to an Apple Store and have them put in a new battery. While you're there, they can fix the OS on the drive. It will only cost $129 (no labor). Make a reservation before you go in, otherwise you'll have to sit and wait for a few hours.
OEM does not mean it's from Apple. Third party batteries for Macs from iFixit, Amazon, or your local mall kiosk are all crap compared to the ones installed at an Apple Store or an AASP.
While the Mac is running, click on the Apple logo in the top left. Select, About This Mac... What OS is being used?
I take it from you that I can't just download a freakin ISO and do a regular 'ol repair install
Well you can if you had a Mac... That's why I keep talking about using another Mac in all my comments.
but I can put the HDD into another compute running Mac OS and update the os on the drive as it sits
No. See previous comment where I mention Target Disk Mode. No need to swap HDDs. Just plug the two macs together and use the fully functioning Mac to run the OS X installer.
So I made a bootable usb with the latest Mac OS using my other mac.
Wait... since when did you have another working Mac?
I downloaded the update from the App Store.
Sorry but your verbiage is confusing me here. Did you do this or are you speaking hypothetical?
A quick google suggests that the reinstall option in the installer environment will in fact do a repair install while leaving all of the data alone.
Yes this is true, but it will also have to download the entire OS in the process. So if you have a second Mac, then just bypass the install to a USB drive and just install straight to the target disk mode MacBook.
10.6 was the last OS that Apple distributed via a DVD. 10.7 and on have been download only.
Also, do not load anything previous to 10.8 Mountain Lion. 10.7 and earlier are no longer supported, it would be like installing XP or 98 on your PC. Just don't bother it's not secure.
TL;DR: Return the iFixit battery. Make a Genius Bar reservation. Pay $129 for a brand new battery and they will even install whatever OS you want (but it will probably have to be 10.8+).