Wow, this looks awesome! I bet this thing barely weight anything at all. And if you collapse the seatpost this should get extremely compact.
Yes, 75mm cranks is a bit long/light for a longer ride. 50 or 45 mm should theoretically give a very nice riding experience on a 12". Well. Apart from the fact that any bump at all is a mountain 
The problem is that the the cranks are not Cottorless/taper at all. They have a constant diameter/diagonal throughout the hole. You will indeed need wedge shaped shims.
Maybe you could try out the plastic shims I designed. It took about 5 mins to print and you might need to cut them a bit into a better shape with a scissor. When you tighten, they get crushed into a mold, and I haven’t experienced any slack so far.
It is also possible to force an ISIS adapter into the hole with a crank extractor. I did this by accident and had to hammer the adapter out with a bolt and hammer over several tries.
When fastening the deformed crank to the hub, it deformed into a Cottorless ish shape and fit quite snugly. I would highly recommend trying the plastic shims first. I count myself lucky I did not brake anything when that happened. 