Thank you for writing back and I am very sorry about your persisting issues. If this laptop is still covered with warranty, then you should be able to have it repaired with your warranty coverage.
As I see it, there are few ways that this can be repaired. But first and foremost, if a Dell technician works on your laptop, please request to FIRST RECOVER your personal files like Word documents, Excel spreadsheets, photos and other personal stuffs. Most tech in the repair shops are just after of making the computer work again without regard to data in it. Better late than sorry, please make sure that the Dell tech recovers the data first. This is very important especially if they will restore the computer into its factory settings. All data will be lost and the computer will work like brand new.
But if the Dell tech will use a Windows 8.1 or Windows 10 DVD installer to perform the repair, then your files will be safe as it is only the Windows installation will be repaired and affected in the process. So better ask the tech how or what is his repair route: If he says he will restore to factory settings, STOP! Recover the data first. But if he says they will use the Windows 8.1 or Windows 10 DVD installer, he can proceed as your data will be safe.
I have already exhausted all the possible procedures to fix your Windows 10 but your Dell installation and drivers always reverts to the faulty profile and the rollback to Windows 8.1 could have been interrupted causing this boot issues now.
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