iRulu U1PRO Android 4.4.2 KitKat root reflash flash stock ROM firmware

Background
My son complained to me recently that there are many virus/malware in his android phone. he did not browse much internet
except regularly check emails and daily websites, two popular games that many peer play, all downloaded from play store.
the symptom: once connect WiFi, it automatically download apps crazily. I tried to install an AV app but no room, had to do factory reset, but once connect WiFi, still the same issue.
the Android system could have been infected
after reset, I installed AVG and Avira, neither helped. installed Avira on PC, connect phone to PC, scanned it,
found and quarantined quite a few malware,issue still the same if connect to WiFi

push volume down+power simultaneously, check: my BB chip is MT6592
Before start, you need tools and drivers:

1. A PC with Windows
2. Your iRulu U1PRO and USB cable
3. Download stock firmware from official website: https://www.irulu.com/download/lists.html, extract it to a folder in PC, let's call folder name stockrom
4. iroot, download from http://www.iroot.com/
5. SP Flash tool from http://spflashtool.com/
6. ADB driver, download from http://www.chinaphonearena.com/forum/Thread-OnePlus-X-guide-auto-install-adb-drivers-fastboot-and-adb-Windows-xp-7-8-10
7. MTK Droid Tool, download: https://androidmtk.com/download-mtk-droid-tool
8. TWRP 2.8.1.0, download: http://www.mediafire.com/download/7ezmib9qr7yyqsk/2.8.1.0.zip
9. MediaTek USB VCOM drivers download: http://spflashtool.com/download/MediaTek_USB_VCOM_drivers.zip

You need to do: Warning: these actions will void warranty on your phone and it only applies to my specific phone model,you assume responsibility for any and all damages caused by following these steps

1.Enable USB debugging mode of your phone in settings > developer options, if you do not see the option: Settings > About phone, tap build number 7 times
2.Root the phone,no guarantee every tool can work, I used iRoot which worked.
3.Back up the phone in case reflash rom fails.install TWRP for MTK, backup the phone whole system with TWRP,
http://www.gizbeat.com/5786/how-to-make-create-twrp-mt6592-mt6582-mt6572-mt6589-mt6595/
Backup with MTK Droid Tool might also work, but I did not try. TWRP backup looks easier to me
4.Backup IMEI and NVRAM, https://androidmtk.com/backup-and-restore-imei-nvram
5.MEDIATEK USB VCOM DRIVERS MANUAL INSTALLATION, https://www.youtube.com/watch?v=sADkzyI8Z5Q
6.Flash the firmware to your phone by SP Flash tool: https://androidmtk.com/flash-stock-rom-using-smart-phone-flash-tool
in step 2 of this link, you remove battery then you can put back.during flashing, I got error:"PMT changed for the ROM; it must be downloaded.", in the SP Flash tool, I choose foramt all+Download, then success, but you need restore IMEI and NVRAM: https://androidmtk.com/backup-and-restore-imei-nvram ,otherwise it can't recognize IMEI/SIM card