Dell Venue 11 Pro (5130 64Bit) BIOS update in WinPE 5.1 x64

I have encountered an issue while trying to upgrade BIOS on Dell Venue 11 Pro (5130 64Bit) in WinPE 5.1 (64-bit). To pre-empt any questions why not use BIOS update option available from boot menu: while it may be quite elegant solution for home use, it doesn't fit well in production environment where the action needs to be repeated hundreds of times, custom audit trail required and some other (automated) post-upgrade tasks need to happen.

After running the executable I receive the following error message:

The subsystem needed to support the image type is not present.

This would point to 32/64 bit application incompatibility and/or limitations of WinPE itself. I have following packages present in my image, as well as CCTK HAPI installed:

--------------------------------------------
Feature Name
--------------------------------------------
SMB1Protocol
WinPE-HTA
WinPE-WMI
WinPE-NetFx
Microsoft-Windows-NetFx-Shared-Package-WinPE
WinPE-PowerShell
WinPE-DismCmdlets
WinPE-SecureBootCmdlets
WinPE-SecureStartup
WinPE-TPM
WinPE-Scripting
WinPE-WDS-Tools
WinPE-EnhancedStorage

Unfortunately I cannot run the update from a "proper" copy of Windows as we need to upgrade BIOS for one of our customers without installing OS (we are shipping devices with default Dell preloaded version of Windows). The way I see it, before I am forced to go down the route of upgrading BIOS from boot menu, I have two options:

- Get a grip of what the exact issue is in x64 PE. Would anyone be able to help out here? Is this something I can fix by adding any components to my PE image?

- Try running update in x86 PE (I believe the executable is 32-bit despite "64bit" string in the name). Currently I can only boot using x64 image as BIOS is set to UEFI mode. I am reluctant to change boot mode to Legacy just to give it a go. Will I be able to safely go back to UEFI without any additional recovery work? I am not intending to change the contents of the HDD. Also will cctk be able to change boot mode back to UEFI automatically, or will I have to implement it as a manual step at the end of the process?

Due to other restrictions and time scales involved Windows To Go is not an option as well. Any help with this will be much appreciated.