*passes out*
I utterly failed to understand who I was messing with. Even after many long and torturous hours in the tank, over many many days, I was unable to successfully construct a position satisfying this harder challenge. I would've simply given up and decided it was impossible, had I not suddenly thought to "steal" your position with the 8 free units, rearrange a few pieces, and then capitalize on all the extra pieces available to make everything work by sheer brute force. Incredibly, it still took AGES, but finally I arrived at the position below.
Fact is, you're just better than me at this - no ifs, ands, or buts about it! And since it's now clear that no amount of effort on my part can ever change that, I won't bother trying to improve the problem anymore.
The one thing I'll add is that I'd be more interested in seeing what you could do in terms of further tightening the restrictions, as opposed to making as big a mockery as possible out of the current ones by maximizing the number of free units. For instance, can the exception I made for the kings be done away with?
OK, 4 better; 5 free units, e.g. +bNb8 +bBa7 +bBb6 +bBa5 +bRa6