Do people avoid Busch Gardens or Disney World when it's hotter than hell
Yep they do - Floridians/locals visit those areas when the not scorching hot.
Link?
You two should go to Busch or Disney in the middle of July. Let me know how long the lines are and the droves of people walking around in the heat at the parks.
Here an attendance graph for the Disney parks:

Heat doesn't seem to be a big factor in Orlando.