[AT&T LG Optimus G] [r.]GimP:
Don't panic, you can always load onto the SD card externally, with a microsd to SD adapter or through TWRP/CWM with "mount usb storage" mode. Failing both of those things I'm fairly certain one of your two phones will be enough to load the SD card for the other phone if needed.

I'm thinking everything you need is going to be the LGNPST route now anyway, and that being said you may as well just flash them both to stock 4.1.2 in that case. First things first, you definitely want to backup before anything gets out of hand, if twrp isn't getting it then pull with adb. Quick breakdown of that below.

The universal drivers straight off the LG site work just fine, but in my experience to get the adb part happy you need to click on your devices, and find your phone, whether it says LG/Nexus4/MTP, doesn't matter. Open it and click on the hardware tab, it should list 3 devices.

unknown (or adb if you're lucky)

Your phone

USB Composite Device

If the top isnt some variation with ADB in the name, double click it, then change settings, it may say drivers not loaded, so on the driver tab we're going to give it one.

Update driver, and it will ask how, the option on the bottom is "browse my comp for driver software" and after that the bottom option again is "let me pick from a list..." I think then it shows a split list, the device type should be ADB something, and if you installed the LG universal drivers then near the bottom you will have "LGE Mobile Sooner Single ADB Interface" windows will complain because it couldn't figure out that its the right driver, confirm you want to use it and it will then show "this device is working properly" go ahead and close all those windows and it will update on the first one with the three devices listed the next time you open it.

At this point you need the adb application itself, I usually just download the whole android SDK personally, but where you get it from doesn't matter. If you're booted in android, make sure adb has root access in developer settings, and debugging is enabled.

open a command promt, for the sake of easy navigate to the folder where you put adb, in windows, or in the SDK example, /android/sdk/platform-tools/ hold shift and right-click on an empty space and select "open command window here"

If android is running, type "adb root" without quotes

and adb will inform you its restarting, if in twrp you will already be root.

Type "adb devices" and as long as its not blank you should be good

again, for the sake of easy "adb backup -shared -all"

and let it do its thing, it should start creating a default file named "backup.ab" it's bound to take awhile as -shared should grab all the sd card contents just to be safe.

Disaster recovery aside, I would use a combination of Titanium backup root, and "backup your mobile" - especially add the second option in addition to titanium, thats one of the only free backup apps that can get MMS as well as SMS, and restore them.

Again, if you're having issue with the computer normally seeing the device, these are still good tools, save to external sd card, remove card and transfer manually to your computer.

ok.. now that all your stuff is *hopefully* now redundantly backed up, depending on the stock image you find, you can either push it via fastboot commands or flash with lgnpst. I know the images are in our forum, most likely in the lgnpst thread. just follow through that and read well ahead before performing each step, you'll be ok.

I believe fastboot can relock your bootloader in all off one command, i don't have the answer if its necessary or not to receive the OTA, but having skimmed the lgnpst thread in the past, I'm positive people have done the flash and then had the OTA pushed from AT&T so they could get up to 4.1.2 for the official JB and partitions. Like I said in the beginning, your call if you want to skip that and just lgnpst to 4.1.2 since you're there anyway. Also, keep an eye out for one of the recent restore to stock / RPM/TZ partition threads, I think shellnutt making a cwm/twrp flashable zip to do the same thing lgnpst teeny bin would without even needing lgnpst.

So, barring a complete hardware failure, dont be scared. Get your data for now, then on to the next step.