"Free will means one can choose between good and evil. If this choice does not exist, how would there be free will?"
So which is it? Is free will possible without evil or not? There's apparently no evil in heaven, according to you, so then there's no longer any free will, again according to you.
Being omnipotent, if God really wanted everyone to go to heaven then he would have never allowed evil to exist. Being omniscient, he must have known that the majority of people would end up in hell but he allowed this to happen regardless. You say that God doesn't want anyone to end up in hell, but he's the one predisposing people for evil and then sending them to hell because of the nature he himself created.
Furthermore, if Adam simply forgot about the rule against eating the fruit then isn't that an innocent mistake rather than a sin?
Free will in Heaven is a choice between all options which are permissible, as the rules in Heaven are not the same as the rules here on earth. For example, alcohol is prohibited in this life but in Heaven it is permissible.
As for Adam, he forgot God's warning, and was sent astray by Satan
The end result is that he disobeyed God's command not to eat from that tree. But God later showed him how to repent, which he did, and God forgave him
As for God not allowing evil to exist if He wanted all people to go to Heaven, this is the case with angels not human beings. Angels don't sin as they don't have a free will to enable them to sin. This is not the case with human beings. We live on earth, and we do have the choice between belief and disbelief, between good and evil, and each person uses his/her free will in making their own choices in life.
A professor wants all his students to pass the course, but does that mean they are all entitled to pass? Or some deserve to pass and some don't? He gives them a test and those who do well succeed, and those who don't fail the course.
Same here. God wants us all to be in Heaven in the Hereafter, and He has shown us what to do, by sending us His Guidance. Those who want to be in Heaven only need to follow His Guidance
It's a personal free choice