It's not cheating, but I still don't like the double factorial. I had an inkling a lot of the other solutions made use of it, and since it's not something those of us who finished formal math for good by the age of 19 internalized, it's much harder to use in solutions that require intuitive leaps. Nonetheless, best use of the quiz tag in a long time!!
I like your double exclamation point at the end of your comment!! I'll argue it's not harder to use in solutions -- I gave an explanation in the puzzle to open up the possibility, and it's a pretty short list of operands that are useful: 4!! = 8 5!! = 15 6!! = 48 7!! = 105 after that, the numbers get too big to be useful. I didn't find a use for 5!! or 7!!, but certainly there are more solutions I haven't found!