Lots of good reasoning on your part. Ankle rig is indeed fast to get to when you're down on your back (or seated, as when driving a car). And a revolver does indeed survive the press-contact shot and keep firing, which is not true of most auto pistols.
However, while a lot of hand to hand fights do go to the ground, a majority of the gunfights I've reviewed have involved distance, with participants drawing their weapons from standing positions. Ankle draw definitely puts you behind the curve in that situation.
Might want to compromise with the LCR in an ankle holster for backup. If the weight of the HK on your hip is causing physical problems, you might want to explore a shoulder holster, or a good pair of suspenders to augment your belt and get some of the pressure off the hips and lumbar area.
Your .38 load is a promising one, and well-discussed here by two of GT's most knowledgeable contributors: http://www.glocktalk.com/forums/show....php?t=1409615