My Malaysian set RN4x is unable to switch on or boot, only red light keeps blinking during charging.  Google and it seems like a common problem but different way of rectifications. So far tried a few but not working.

After trying the different self diagnosis suggested by google, my 18months old RN4 still cannot be charged or boot. Brought to Xiaomi Authorised service center to diagnose and was told problem with mainboard, so not worth repairing n paid rm25 for that. My 3year old RN3 still kicking strong.

The xiaomi authorized service center said my phone is gone and  data not retrievable. But when I brought to another shop, tested my battery ok but still can't charge and can't switched on, dismantled the battery n powered d phone directly by charger n phone can be switched on and booted, everything still intact felt relieved.
At least now allowed me to savage my data. Will backup all data b4 repairing the shorted mainboard, repairing cost btw rm150-rm250.
 
Useless  xiaomi service center charged me rm25 for a lousy job.

WTA:
1) Is it safe to continue using the shorted phone with direct power while backup the data and reset it to factory setting?
2) Is the repair cost of 150-250 which takes 2 hours too high?
3) Normally would repaired unit last >1 year?
4) Where to sell it as scrap since the screen n battery still working?


Any opinion is welcome.