That's recovery, I'm talking about download mode (needed to flash a kdz/TOT [and "
just maybe"] a few other tricks)

.
Turn off phone. Hold the Vol+ key and plug in the USB cable from the phone to your computer. I think getting to recovery is nearly the same, only using the Vol- key instead?
OK great! First & foremost, make sure you have the phones drivers, and adb working correctly and can access the device.
Easiest installation method for Windows is here. It (optionally) installs drivers for nearly any device, and adb/fastboot is installed system-wide for use in any folder. USB debugging enabled on the device, connect via USB cable, and allow+remember access from the Computer. Open a command prompt in your working folder (ex D:\Realm) and type adb devices. This should present you with the serial number of the device if you have everything setup correctly, and have accepted connections to the phone.
Download the OTA
but don't install it yet. With a ROOTED DEVICE, use some root explorer.
ES File Explorer File Manager is free and works guite well, but you have to enable root explorer in the settings (SU will prompt you to allow it). Browse to /cache folder inside the root folder of the phone and copy (or move if not going to actually install yet) the downloaded ZIP file to your emulated SD card (or your EXT_Sdcard), then copy it to your PC.
Upload this file somewhere we can grab it (Dropbox, Google drive, whatever) and give us a link. We'll rip it open to see what it contains and maybe come up with a solution. No promises of course, but I'm willing to try. If you don't want your dropbox, google drive (or whatever) getting hammered by the lurkers, start a PM (Conversation) with bcrichster and myself linking us to the file. As bcrichster knows from our other Convo's, I've been working a lot lately, and my time has been quite limited. Collectively, we can share the workload and/or explore different methods once we know what it contains.