I can't answer all your questions but I have just bought some shade cloth and believe you will need a 45-60% shade type. It is my understanding that you want to install the cloth on the outside to prevent the sun from penetrating the greenhouse. The shade cloth should be mounted off of the roof so I plan to lay a pipe 10 inches high across the center of the greenhouse frame and attach my cloth to that. The northside of my greenhouse is insulated so I do not need to put shade cloth over that, although I may. As far as vents are concerned I was just going to make a little frame to hold the cloth away from the vent if need be.
Right or wrong, my greenhouse is 10x16 and I ordered a piece of shadecloth 20x10.
I've attached a link that may help.

Here is a link that might be useful: FAQ shade cloth

i was at the local nusrsery yesterday and she recommended to me to buy a white shade cloth

"Is it better to install the shade cloth inside or outide the greenhouse?"

Either way. Most shade cloth is dark and will absorb heat and therefore people will recommend putting it on the outside. Mine is installed on a frame on the inside so that I can easily put it up and down and protect the cloth from wind damage. I use Aluminet, which is reflective...it absorbs less heat...great stuff.