Boot Camp Catalina / Windows 10 Weird Driver Install Error

I have spent 3 days trying to sort this out, including 3 calls to Apple Support, who were really nice but ultimately couldn't help... I have:

15" Mac Book Pro mid 2014 with a clean install of Catalina and the latest 64bit Windows 10 ISO from Microsoft (1909).

Running Boot Camp Assistant, everything configures fine with Win 10 and Boot Camp Windows Support 6136 being written successfully to my 16GB USB. I had to split the 4GB Windows install file in two because of the FAT file length limit, but that has worked fine.

Windows 10 then does its install, BUT during the install Boot Camp does not automatically install its drivers and support software. So, I finally boot into Windows 10 without any network connection (no WiFi drivers) and Microsoft basic video drivers etc. Ok, I thought, I will just have to manually click on the Boot Camp Setup.exe to get that sorted.

BUT upon double clicking the Setup.exe I get a Windows error "The file or directory is corrupted and unreadable". Listing the path to the Setup.exe file. Same if I double click the nVidia drivers setup.exe. So, I try again with a different stick. Same error. So then I copy the Boot Camp folder from the 'stick to the Desktop. No errors during copy and checking various text files all appear fine. Double click Setup.exe - same error.

Right click Setup.exe and Run as Administrator, alert asks if I want to trust Apple (so its reading that its an executable OK, recognising the files header etc) I say yes and the error now says "Windows cannot find {full path} Make sure you typed the name correctly, and then try again" - listing the correct path to the file that I just double clicked and Windows reported was an Apple file so its definitely there!!!

Thats when I called Apple. Since then I have re-installed macOS and gone through the Boot Camp download/install process 3 times, including trying Boot Camp Windows Support files downloaded via High Sierra - all the same, the Boot Camp Setup.exe doesn't work.

Apple's only solution was to manually find and install the drivers myself. Which I have done via the Boot Camp files on the USB stick as some of them work fine (Wifi works with the Broadcom Win8 driver for example). Using them I have sorted the Broadcom WiFi, nVidia graphics, Apple touch pad and camera.

The only thing missing is the Boot Camp Control Panel that is normally accessible from the System Tray (bottom right thingy). If I try to run the installer for that I get an error saying "This application requires elevated privileges. Launch the installer through setup.exe" which is my stumbling block. I have also tried running this from the command line with msieexe.exe and /a .. but that returns a 2203 which is probably due to the lack of privileges. Really pain is that I need the BC Control Panel to enable two finger right clicks on the track pad.

Anyone any ideas how I can manually install the Windows Boot Camp Control Panel? i.e. how I can gain the required privileges. Or, ideally, any idea why the Setup.exe isn't working in the first place! - all very odd. Originally I thought there might be confusion from my Time Capsule backup (El Capitan) being restored into Catalina before the Boot Camp process began. Thats why I formatted (erased) the SSD and installed a fresh, clean, Catalina but the error persists.

I am guessing that its not compatible with Windows 10, but surely everyone would have the same issue so Apple would know about it, and clearly they haven't a clue.