Mike,
I'd argue that the reason that RM got it right has more to do with Russell than anything to do with Mackenzie.
Many of the fairway bunkers at Kingston Heath also suffered from being too close to the tee, where most of the carries were no more than about 150m-160m. Holes 2, 6 (rhs), 12 (from the original tee), the original bunkers on 13, rhs 17 and the original traps on 18 were all short carries. Although it appears Mackenzie did not specify bunker distances on his sketch plans, the original layout put down by Vern at KH appears to have faithfully followed McKenzie's bunkering scheme.
At RM, there were considerable revisions and amendments to Mackenzie's layout. The acquisition of Bumpford's block and the decision to build the East course meant that holes 1 and 2 were moved northwards and 3W was shortened from the holes which MacKenzie laid out. The 4th tee could then move back making a longer carry over the bunkers built into the crest. MacKenzie's 6W had no fairway traps and a green which was to be angled to favour an approach from the left. Holes 10, 11, 17 and 18 were Russell/Morcom holes. Accordingly, the only fairway traps which MacKenzie included on his sketch plan that are relevant to any argument as to correct bunker positioning are 8W and 12W.
In his course designs, Vern appears to have remained loyal to Mackenzie's bunkering principles employed at KH which, by the mid 30s, were outdated. By contrast, Russell appears to have had more foresight (perhaps due to his experience as one of the leading amateur golfers) in designing doglegs and placing bunkers which were relevant many years after he laid down his courses.