Malware automatically installing unless I disable the play store
regarding xHelper


unpack firmware247.com/sky-elite-6-0l-plus-firmware  
install VCOM preloader usb drivers  
install Mediatek SP Flash Tool  
load the scatter file, DESELECT all checkboxes (especially preloader)
switch off device completely  
flash boot, system (click Download only, Download, then connect usb-cable)  
flash cache, userdata (= factory reset if you wanna be 100% sure)


regarding OTA updater  

readback boot.img with SP Flash Tool, patch with Magisk Manager, flash magisk_patched.img into boot, root the device and disable the OTA updater from terminal or adb shell  

su
pm disable --user 0 com.adups.fota
pm disable --user 0 com.adups.fota.sysoper
pm disable --user 0 com.fw.upgrade.sysoper


after this, restore the original boot.img (unroot device) and uninstall Magisk Manager  

(Note: unlocking bootloader is most likely not required for this old device, SP Flash Tool can flash with locked bootloader. However, be aware unlocking bootloader will perform factory reset)