boot – Grub/Bootmgr issue with Linux mint 19.2 and Windows 10

I have been dual booting Linux mint 19 and Windows 10 on my laptop for 3 years with no problem. Today, I ran an apt-get upgrade command on Linux, and had grub, os prober etc. authentication errors. Purging grub/* and reinstalling grub-efi fixed the package upgrade problem. However, after booting into Windows, I cannot boot into Linux again (grub screen does not appear, laptop boots into Windows directly)

  1. Following several forum suggestions, I ran

    bcdedit /set {bootmgr} path EFIubuntugrubx64.efi

with admin privileges from Windows cmd. Now, restarting the laptop gives “Selected boot image not authenticated” error.

  1. I turned on legacy boot from the BIOS. This made ubuntu back up (with no grub screen). From the ubuntu terminal, I ran
    sudo update-grub
    The terminal display showed that grub was able to find the Windows Boot Manager image. This time, I rebooted, the grub screen was shown, but there was no Windows option on it.

  2. I booted into ubuntu and ran boot-repair with option “Backup and rename Windows EFI files” on. After all the changes are applied, the Windows option appeared on the grub screen, but pressing enter on it just brings me back to the grub screen. Opting F11 from BIOS (system recovery) also brings me back to grub. I haven’t found another post with the same problem here. This step was also all under legacy mode. Turning off legacy mode gives image authentication error and nothing can be booted still. The paste bin is here: http://paste.ubuntu.com/p/96jHFRfYBF

Any suggestion would be greatly appreciated. I would do anything to recover my Windows system at the moment (including purging the Linux Mint system, since my Linux filesystem is more organized and I was able to back everything up during step 2).