As for multiple coats of Pt, I've found it completely unnecessary if you're using a typical high quality paper for printing.

In addition to David's work, look here:

Also see the old View Camera article by Stuart Melvin, July/August 2001, The Pigment Over Platinum Print.

I would say that yes, you have fine control over the gum, but I see it as similar to a flavoring or spice added to a hearty stew, I don't find very fine control necessary to spice things up. I sometimes see some softening, but I'm sure that's due to my sometimes sloppy printing. It's WAY more forgiving in that respect than multiple exposures in Pt.