i wanted to follow up with what i've found over the last few days.
i did find a solution to convert 3mf to stl. it involved building the software myself, from source. lib3mf and its examples are on github:
however, there are no build instructions, so i imagine that for anyone without significant experience building c/c++ applications, this is a genuine challenge. for me, their build setup seemed really janky. soo.... ymmv
i did find another 3mf-->stl solution in some forum somewhere, but since the source was in a mystery .zip, i did not try it.
that makeprintable.com site works ok, but not great. as a mathematician, i tend to push the boundaries a little. my models are generated from numerical data, and have to be post-processed significantly to be printable. makeprintable currently (as of 20161001) has two problems for me. 1. increases the filesize dramatically (like 250k to 10mb), and 2. in one of the two tests i've done, screwed up the geometry. so, i consider them a non-solution so far. other people may have better experiences than me, and i have left feedback with them to help identify where their software is making mistakes.
native 3mf support still seems like a reasonable thing to integrate into Simplify3d, especially since lib3mf is open source with a commercial-friendly license, and there's an example of converting to stl on the web provided by them. should be a one-day fix for them...