David
Glad I could help you in your quest! Sometimes such questions can be answered with the help of a good reference library (i.e., having an idea of where to look) and I can take no great credit for anything but a good library.
It certainly would help if you could actually handle such a shell and see what the hefting actually meant. Many years ago, I made a point of using the Boer Mauser and the contemporary Lee Enfields on the range with good ammo to see what they really were like compared with modern rifles. Fascinating to actually see how formidable both rifles were and how well they handled.
If any acknowledgement is to be made, I think it should simply be to the site and Mr.Biggins. If you wish to contact me further, you could always PM me with email details, etc.
In summary, I suppose it would be possible for one man to set the fuze, load and actually fire the 12 pdr RHA gun. Bear in mind that he also would have to align the gun!
Have you read Rudyard Kipling's UBIQUE?
Regards
IL.