12-foot or 18-foot ceiling for 10-foot trees?

I plan to construct a large attached greenhouse as shown in the image above in an attempt to grow tropical fruit in zone 6. I will have fans, a heater, and hopefully some type of shade option. The scale of the image is pretty accurate. The way the windows of my house are positioned, either a 12' or 18' height works.

Assuming all dwarf trees stay under 10' of height, which height is optimal? Is 2' of headroom too little, making proper airflow impossible? Is 8' too much, making climate control even harder?

I am also wondering if the canopy will be significantly wetter and hotter on the higher northern side than the lower southern side, or if placing the fans on the upper northern corner of the east and west walls will minimize the difference in microclimates.