[General Questions and Answers] alecxs:
I read this and just think, what the hell... i can not understand why "/data" is write protected. Just some thoughts running through my head:

- phone is not rooted and completely stock, but factory reset not working!? in this case ask for warranty and send it back to seller!

- if no warranty, ask Huawei for support. don't tell much, just claim factory reset is not working.

- ask any mobile repair shop, they have special hardware like Octoplus Box and can do factory reset.

- if possible - make a "backup user data" with Android system recovery, you may restore it on working phone, later.

- if you have read-access to "/cache/recovery" look for file "last_log" and copy to sdcard. "/proc/emmc" and "/proc/partitions" are another useful files. keep this in case you have to manually create the scatter file.

- you can use MediaTek SP Flash Tool Readback function in download mode to make a ROM backup dump. for readback you need to install VCOM Drivers, then first click Read back, then connect the phone to PC while switched off and insert battery. MTK Droid Tools will then help to segment the partitions from it. or readback every partition separately with scatter file. the most important files are "nvram.bin", "preloader.bin", "boot.img", "recovery.img".

- if phone is locked, ask Huawei for unlock code (not sure if this works on broken phone). there are different kinds of S-ON some will lock only bootloader, some lock all partitions, too.

http://www.modaco.com/forums/topic/362724-official-code-request-to-unlock-the-bootloader-final-post

- try fastboot mode factory reset (not sure if working, just googled this stuff i found).

http://www.hardresetandroid.com/hard-reset/huawei-hr/huawei-ascend-y520-hard-reset-code-format-solution

- once you have unlocked the phone, do factory reset. phone should boot now. its hardly imaginable that "/system" is damaged because of never rooted the phone.

- if still not working, you should now be able to flash custom recovery, too. be careful, select tested binary for your special phone model only! flash in fastboot mode. avoid flashing with SP Flash Tool as long as you not absolute sure that scatter file is 100% correct (MTK Droid Tools may give you wrong scatter files for devices > 4 GB!)

- from custom recovery, you should able to root the phone via adb root method or install the "supersu.zip"

- once you have root, you can mount data partition read-write from adb shell (in recovery mode) and format or delete it. of course the custom recovery has a wipe data option, too.

- installing aroma file manager zip is another way to browse and modify everything in recovery mode. you can copy files from similar phone into "/system" folder, if phone still not working properly.

- don't restore a full backup from similar phone (especially NVRAM folder), this may clone IMEIs and WiFi MAC address.

- you can install a stock rom zip from custom recovery instead, and use my script bckp2cwm.zip to convert the user data backup into cwm format and restore "/data" folder only (converting script requires linux pc).

- delete suspicious apps from "/data/app" + "/data/dalvik-cache" + "/data/data" (every app has three entries with similar name). do it in recovery mode with aroma file manager, or on linux pc from backup file during converting (at this point where script is waiting for user input).

- search and read for "unbrick guides" on xdaforums.com, as long as it is not a physical damage (Vol keys not working, emmc wearing out, etc) there must be a solution to get this phone working