Did you (since your article) do an exhaustive search? In your article you
mentioned that you had 6 positions that still do require 18 moves. And you
mention that you doubted that there would be 17 move solvers. Have you
proven since then that it can not be done in less than 18? If not, send me
your positions and I will try.
I have done an exhaustive search and none of the 6 situations of 18 moves
could be reduced to 17 moves (within the group of <R2,L2,F2,B2,U,D>).
For the case you want to verify, one of them is:
L2 U R2 B2 U2 B2 L2 D2 L2 F2 D' L2 B2 F2 L2 F2 U' D