Over two years later, I've decided where to go with the valve gear and actually made some slight progress.
I'll stick with the Stephenson's gear, but have redrawn the set up with lifting links positioned outboard of each expansion link.
The original problem with inboard lifting links was a foul with the boiler underside. As now positioned, the boiler can sit at scale pitch without fuss.
A weighshaft/ lifting arm assembly was silver soldered up. The main shaft is a hollow tube, cut to fit between the frames, and the assembly rotates around a solid rod. This rod slides in from one side and sits in holes within each frame; it will be trapped (probably) by slide bar brackets when they are bolted onto the frames in due course.
Here's a snap of the LH components in situ and in full forward gear, with temporary pins to hold all together.
The frame cross member is milled from brass and supports each valve rod through a bush. (Well, just a hole really).
The expansion links were machined, with silver soldered brackets for lifting links. A straightforward turned bobbin acts as a die block.
As can be seen, although prototypically Stephenson's, no attempt has been made to scale the components. In fact, they are quite crude but will hopefully do the job well enough.
This is the RH gear, looking outside in.
The overall idea of producing a 'garden railway' type loco for some standard gauge G3 live steam, R/C shunting remains.
A Roundhouse type gas fired boiler with standard Roundhouse regulator/ gas/ etc components and a Slomo between the frames completes the spec.
The boiler could have been a standard Roundhouse item, but there is some extra length available so a slightly longer boiler is being produced professionally to take advantage of increased capacity. All other items will be bought off the shelf.