My solution was to source a bar-action hammer-gun rather than back-actioned examples with the problems described.
Trouble is, by the time the back-action lock was superceded by the bar-action, the Jones under-lever was already a thing of the past and the top-lever was king.
Under-lever bar-actioned hammer-guns are rare as rocking-horse sh!t!
Top-lever howdah pistols are a problem because the plod tend to regard them as sawn-off shotguns, and in any case they put the web of the firing hand at risk.
I have sourced a back-actioned hammer gun with a side-lever that should do the job.
It is 12-bore and I will need rifled 16-bore inserts, slow twist for round ball, to complete the project.
This process will change the firearm class in my jurisdiction from "A" (shotguns & rimfire) to "H" (handguns) and I can't do that on my basic armourer's licence so would need a gunsmith, presumably with special permission from the plod.
I can do everything up to cutting the 12-bore barrels to pistol length.
Unsurprisingly, the project is on hold for the time being...

The examples of original howdah pistols you guys have posted above are marvelous things and an endless source of inspiration!