Default [Q] Galaxy S3 L710 stuck in endless "gapps has stopped" loop, how to reset?

I am having problems with my Samsung Galaxy S3 (L710, Sprint). I updated it to Android 4.4.2, which disabled the ability for the phone to write to the card. This meant that several apps, including Spotify, Swiftkey, and the camera, were all unusable, as they all defaulted to SD storage.

To fix this issue, multiple forums and sites recommended downloading an app called SDFix. The rooting process took several hours and required a system reset afterwards, but it is rooted now. Apps such as Root Checker said that the phone was rooted properly, but the SDFix did not work.

The next step I took was to try to flash an older 4.3 ROM to the phone, as it would still have SD capability. I wiped the cache as well as the Dalvik, but the installation of several roms, both stock and CM, have failed with the error:
Assert failed: getprop("ro.bootloader"? == "l710vpbLj7"...
repeated for about six other file names, which I think are different bootloaders.

Now when I turn the phone on, it looks like it has been reset, but I get the notification "unfortunately, com.google.process.gapps has stopped", which appears again immediately after closing the first one. It is an endless loop and I cannot do anything in the phone. Is there any way you are able to help me?

Thank you.