EFI Partition Corrupted

Question

I've got my hands on a laptop that went down several linux installation and has now a corrupted boot configuration with a mix of EFI/MBR grub configs.

For starter when I turn it on it says:

Booting in insecure mode
Failed to open \EFI\Microsoft\Boot\grubx64.efi
Failed to load image
Failed to open \EFI\Microsoft\Boot\MokManager.efi
Failed to load image

The only way I can boot is by pressing esc to enter the "Startup Menu". From there I select F9: Boot Device Option that propose me several choices.

1. OS boot Manager: I go back to the first screen and I'm stuck again.
2. Ubuntu: it launches a grub session
3. kali: the correct and only os I could start
4. Boot from EFI file: it asks me to manually select the efi boot file I guess

Only selection 3 is the one that lets me use my OS. Once there I can see a kali version of the grub menu with several options and I can launch my OS.

To fix this I've lunched a live session of boot-repair-disk but the standard procedure proposed by boot-repair did not work at all, also the advanced options grub location, grub options and so on are all greyed out and I can't select them.

I manually mounted the /dev/sda1 to /boot/efi. In here I can see 4 dirs:

Boot/
    bootx64.efi.grb # empty 
    bootx64.efi
Microsoft/
    Boot/
        bootmgfw.efi
        bootmgfw.efi.grb
        bootx64.efi
        bootx64.efi.grb
kali/
    grubx64.efi
ubuntu/
    grub.cfg
    grubx64.efi
    MokManager.efi
    shimx64.efi

Kali is the only active and installed OS, all the others do not exists anymore.

I'm quite confused, how to fix this?

Note: I'm not planning to reinstall Windows in this machine.

Edit: I have a total of 3 partitions, and my kali installation sees them as follow:

/dev/sda1  --> /boot/efi  fat32 512mb
/dev/sda2  --> /boot      ext2  244mb
/dev/sda3  --> Os Disk

I should probably get rid of /dev/sda2 as I did not create it?