Oh my goodness I managed to avoid the factory wipe upon sideloading! I never believed this would actually work, but it did!!
The trick is dead simple: Right after all the system files and extra partitions (bootloader, modem, etc.) has been updated, during that tiny pause of 2 or 3 seconds between the message
"script succeeded: result was [/system]"and the wiping process, I first pressed and held the
"boot to recovery" combination (Power + Home + VolUp) and noticed that doing so had
prevented anything from going beyond this point; I then
added VolDown and held till the phone eventually rebooted and
showed all green upon partition check shortly before booting the the lockscreen, just like after applying a normal OTA update. After the launcher had shown up, a message was shown in the notification center saying "Android is updating...".
Just wait till that finishes and you're done! I'd almost burst into tears seeing my data being just fine. See the pics in the attachment (ignore the one hour between the two updates, I was trying to rewrite this post after I accidentally got logged out of the forum and losing everything I had written in Incognito Mode).
So again, these are the steps:
1. Boot to recovery apply OTA zip file via adb sideload;
2. Wait patiently through zip verification, patching system, updating other partitions (modem.img, boot.img, etc. It goes quite fast from this step onwards, stay awake!), removing unneeded files, copying new files until it shows: "script succeeded: result was [/system]";
3.
Immediately press and hold Power + Home + VolUp + VolDown. Have your fingers already at the right places, since the pause before the actual wiping process begins is not more than 5 seconds;
4. You'd see a screen flashing through in the split of a second, showing all green for the partitions updated;
5. Wait till the phone boots up;
6. Unlock the SIM and pass the lockscreen. A message showing "Android is updating..." would show up in the notification center. Side note: if you go and check the "Status" screen in Settings - About phone during this process, the ROM status would show "Custom".
Don't worry. You'd see it turns
live back to "Official".
Hope this trick could help y'all!