[Q] How can i make my Zenfone 2 (ZE551ML) system looks factory brand new?

Hi, Few months back i bought the Zenfone 2 and now i want to sell it because it's breaking apart (Fast charging not working, no vibrate, etc), but before that i want to make the phone's system to factory system (without TWRP and non custom ROM), "well why don't you just reflash the whole thing?" reason i make this thread is because i tried installing the latest WW firmware for the ZE551ML using TWRP but it says it failed to mount or something then 5 minute the whole thing failed, that's one thing, second problem i accidently pressed reboot system when the flashing failed i then tried to turn it on to recovery again it doesn't work. FYI, I didn't make any stock backups because i thought everything wasn't gonna go wrong since i see people have no problem doing it(flashing roms and stuff)

Question:
1. Do i need to install the latest lollipop firmware then update it to marshmallow? or i can do it manually using SD card by renaming the file to MOFD_SDUPDATE.zip ?
2. Can i flash stock recovery without PC?
3. How can i reflash the whole thing if i can't get my phone to recovery ?

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