Just to report I seem to have finally breathed a bit of fire back into the MSTS editing process, on the third attempt.
Have succeeded in getting the editor to run reasonably stable to allow me to lay @ 13 miles of track on a short test project, though as we all know that was always the easy bit. The key seems to have been leaving things as vanilla as possible, i.e. at this stage not installing X-Tracks, Scalerail or any other custom track sections that inflate the t-section. The only thing I haven't yet sorted is keeping the RE window re-sized as despite changing permissions in the registry and amending the relevant entries the changes are still not saving. It's only a few seconds work each time though.
Will report more in a couple of weeks if things are still going well and maybe some screenshots of the WIP.