1. Once in Heaven, can you sin? If not, you have no free will.
Heaven is the absence of ego. Where there is no ego there cannot be sin.
Where there is no ego there is freedom. When we make choices from this freedom all choices are Gods choices and the results will come according to Gods plan. You cannot judge them. Heaven is not only after death. People can experience here on earth if they are free of ego.
2. If yes, can you be banned from Heaven for having sin? If yes, then salvation is not eternal and chances are, one by one, over an eternity, we will all be banned
You cannot be banned from heaven. Salvation is not something you get and lose. Salvation is to wake up from our ignorance and realize that our deepest reality has been already in God and will be there forever. For God creation and salvation are not two things. For God creating is saving.
3. If no, then God is in for a very rough ride as everyone can curse him with impunity.
No one will curse God but everyone will bless God for his or her unconditional love, compassion and mercy
Since God is all-knowing, and surely knows the answers to 1, 2, 3, then what was the point in creating the universe?
Sufi mystic said: I was the hidden treasure and wanted to be known so I created the universe. In a way God is discovering his or her infinite possibilities in and through creation. Creation is nothing but one aspect of the divine. God manifests himself or herself as creation in order to unfold his divine attributes of love, compassion and mercy. Creation is a kind of divine play-divine leela, divine love, divine sacrifice, divine song and divine parable or divine joke, whatever you like to call it.