RE: Getting the green house ready to go
I have never used as small a greenhouse as this, but do have experience with ones ranging from 30'x120' and larger range. The larger it is, the easier to maintain the desired interior environment.
You could add roof vents that open/close automatically to exhaust excess heat. Very simple addition and operation. You would need ground level venting as well to facilitate air transfer.
But this, as well as utilizing the fan, will increase the difficulty of maintaining humidity as drier, cooler, air from outside is drawn in. Although cooler air will help with that once the temperatures are regulated. Kind of a Catch-22 situation.
Some 40% shade cloth covering the sunward exposures will reduce heat as well, and you can still have adequate sunlight for good growth.
As for humidity, not sure. That small of a space could be difficult, and/or the most expensive part, of the proposition. Most grow operations I have been around use commercial equipment for that.
Once it has plants, the transpiration rate and the additional moist soil will add some assistance as well.
Hope you find solutions! I'm sure it can be done.