Peter, yes, Martins slide bar assembly is tried and tested and works very well, the only down side I found was trying to file the cusp inside where the cross head slides. By flipping the lamination's 90° the cusp is on the outer edges and much easier to clean up and leaves the working surfaces already perfectly smooth.
On the W1 the reversing lever crank can be moved to which ever position you choose, mine has just dropped to full forward due to all the handling.
In reality the engines had a clutch mounted on the cross shaft which locks the shaft in place or reduces the risk of it adjusting the cut off on the move.
The pins for the expansion link are always going to be hard which ever way you decide to develop the etches, on the W1 I went for pins in the expansion link (much like you have) and then made the motion bracket double lamination's to thicken up the boss area and reduce the need to add washers on the inside.
I think the reversing rod is fixed on the A3 kits, you'd need to remove the half etch footplate bracket for starters and fabricate a new one attached to the footplate. The tail end where it goes inside the firebox wrapper should be long enough to support that end and as long as the front end is pinned to the reversing lever crank it should all work.
Where it falls down on the A3 (and W1 I have to admit) is when you try and take the body off. Right now I cannot think of a way round that for both kits, or any LNER pacific truth be told.
MD