Mark,Bill I think you described the rear bogie of the R perfectly. Although I am not yet concerned with its derailment abilities, I have noticed that it is very difficult to rerail, particularly on a broad curve. As you put the 4 wheels on the track invariably you drag the rear drivers or all of them off the track. on my 33" minimum curve out of the bottom station, the bogie seems to be at maximum deflection. I tugged on the trailing 4 wheel frame gently and the driving wheels slipped straight off as if they were already under tension. I could not easily get all wheels back on and the trailing truck appears to look out of alignment.
Without trawling back through this long post, I do recall somebody mentioning earlier that the 6th axle of their R class was sitting too high above the rail. I didn't bother making any comment at that stage.
However, it brought to mind some modifications a mate and I found necessary to do to a few of his brass models of US outline locos. These locos had 4 wheel rear bogies and he was having problems with derailments when reversing. After extensive slow-motion testing through turnouts etc, I determined that the only way to cure the problem was to perform major surgery to the bogie/s.
In each instance, the troublesome bogie was of the type that had a pivot lug which extended forward of the bogie and was attached to the loco via the rear-most screw that held the drivers' keeper plate in situ. As such, the arc through which the rear bogie travelled, was not allowing the 2 axles to maintain proper contact with the rails.
The fix was to chop off the pivot lug and fabricate a short drawbar. This pivoting drawbar was attached to the centre of the bogie and extended forward, to be attached to the original mounting point screw. This allowed the bogie to move more freely in a lateral plane. Given that these bogies were metal castings, the weight was sufficient to keep them on the rails.
If you do a similar thing to your R class, I suggest you would need to add some weight to the bogie. This assumes, of course, that the bogie is attached as I described above.....it sounds to me as though it must be if the front axle is sitting above the rail.
I think your suggestion of cutting the lead fixing off and rebuilding the attachment seems to me an excellent idea. I think I will give it a go