Vicki this is so wonderful! You are going to just love growing your orchids in there. The shade cloth question: Only 30% of light gets through 70% shade cloth. You will want to use 52% shade on the top and 70 % shade on the west side (at least that is what we do here and we have about the same type of weather (104 today on my back porch!))
The way we solved the humidity problem when using the 'Swamp Cooler' (Kool-Cell) is that we installed an air conditioner to remove the humidity and use the Kool-cell to add in cool air and cool the place down. We even tried adding a chiller (for huge reef tanks) to the water tank of the kool-cell, but it did not make enough difference to keep using it.
Keeping the ground cool (wet) will help with cooling too. Today for instance, I had to go and spray down the gravel floor in our greenhouse. It raises the humidity and helps cool the space a bit. I can hear the exhust fan running full on right now and it is 10:20 something at night. Still is hot in there. We also added another exhaust fan to the side of the greenhouse. (So one in the very top (24") front to remove the heat from the rafters in the greenhouse and one on the side at plant level 30" to keep pulling the air from the kool-cell across the space.)