Windows 10 Boot Manager Fail (endless startup loop). Certain files are locked/cannot be deleted, restricting system restore. How to fix?

(Only asking this here after reading this and checking out other relevant blogs/forums and asking there as well)

I am facing an issue with my Windows 10 on my laptop (Lenovo Y720 15-IKB). The boot manager does not start, there’s an endless loop of restarting and displaying the Lenovo screen again and again. To resolve this issue, I created a Windows 10 bootable USB to access repair tools etc. to see if they could help or not. I have already tried the following:

1) If I go to Startup Repair, it “logs” an error in the SrtTrail.txt file that there is a version mismatch and hence Startup Repair could not repair the PC. Most probably its because I had the latest Insider build installed and the bootable USB was of a slightly older version. I wonder if I can get a .iso of the latest build to resolve this issue.

2) If I go to “Reset” system, it says that the drive where Windows is installed is locked. Please “unlock” the drive and try again. How could I possibly unlock it, when it won’t even start. I can’t access the safe mode either.

3) I had created restore points earlier, to which I tried restoring to however, I received an error code 0x80070091. Afterwards I googled around and tried a few scripts (similar to those mentioned in this link https://www.easeus.com/backup-recovery/system-restore-failed-and-did-not-complete-successfully.html) and the error changed to “An unspecified error occurred during system restore (0x80070780)”.

4) I finally narrowed down the problem via Command Prompt (when starting using a bootable USB and going to command prompt) to certain files and folders which “can’t be deleted”. I tried pretty much all techniques I could think of. These files were in the folder “C:\Program Files\WindowsApps”, one of them belonged to MS Office and another related to Lenovo. I have tried the following:

• Tried deleting the folder (and individual files), upon trying to
  delete a folder I get the message “The directory is not emply” and
  upon deleting a file I get “The file cannot be accessed by the
  system” and for other similar commands I also got an error saying
  “The file is being used by some other process”.
• Tried renaming the folder
• Tried changing access permissions (using takeown
  command etc. and retried the above)
• Tried moving the folders to another drive so that System restore could work for the C: drive
• Tried opening the folder etc. None of the above seems to work.

5) I have also tried “chkdsk” and “mbr” related repair strategies, none seem to work. Apparently, all is good based on the result of “Check Disk”.

Assuming a “Clean Install” is out of the question, what else could I do to repair / restore to a restore point?