1. At what point did the beings in heaven stop having free will?
Some point before the original sin.
What caused this to happen?
It was revealed to the angels the humankind will fall and God himself would become a human. This implied angels to worship the human nature in God. Some of them didn't like that and rebelled.
2. What causes a person's free will to be removed when they go to heaven?
Total revelation. Total revelation means instant discernment and the vision of God. When a free-willed being sees God he can't anymore choose (between God and evil). Just like you don't have free-will between choosing a ton of gold or a pin. Free will needs natural laws and total privacy, that's why nobody can read your mind and why God is not totally revealed to us (almost hide).
Are God and Jesus eternal zombies too, lacking free will?
No.