What is the reason for the windmill's first collapse, and how many times do they have to rebuild it?
As the previous point notes, the windmill was built once and re-built twice. The first time it collapses is an important moment for Napoleon to cement his power over the farm and to blame Snowball for something terrible in order to make sure no loyalty to Snowball remained. It is one of his first truly effective forays into propaganda and he seizes the opportunity with gusto. Once he blames Snowball for the collapse of the windmill, Napoleon quickly sees the utility in blaming Snowball for everything he can in order to show how smart and wise he is and how he was the rightful leader all along.
The battle with the men is the second time the windmill is destroyed. Having occupied a portion of the farm, the men place dynamite in the walls and blow it to smithereens. This destruction actually drives the animals into a great frenzy and they drive the men off the farm. They again begin the task of re-building the windmill.
Of course, once the windmill is finished, it is not used for the promised reduction of labor and increase of comfort but is instead used to enrich the pigs.
The first time that the windmill collapses can be found in Chapter 6. In that chapter, the reason for the collapse is that the animals have built the windmill with walls that were too thin to hold its weight up. This made it collapse in the storm. Since that is pretty much his fault, Napoleon lies and says that Snowball has sabotaged the windmill.
Overall, the windmill has to be built three times. This was the first time. After they rebuilt it, there was the Battle of the Windmill where the people destroyed it. The animals rebuilt it after that and that time it did not get destroyed.