Fair question and a good conversation.
Two quick thoughts, before my wife becomes too angry with me for being on the internet all day:
First, your scenario assumes that in heaven we will experience time in the same linear way in which we do now - we enter into heaven and then, should we still have free will, we may or may not choose sin. But, as far as I know there is no guarantee of that. I find this verse interesting:
Revelation 21 (NIV) said:
4 He will wipe every tear from their eyes. There will be no more death or mourning or crying or pain, for the old order of things has passed away.
Not every translation uses the term "order of things" in fact most translate this as "first things" or "former things", but every translation seems to say something about time occurring linearly. i.e. that something could happen before something else. I just found that to be a very interesting thought; that is that things might not occur in the same linear "order" as which we may be accustomed on earth. The truth is that none of us can really know the exact specifics of heaven until we get there, but it could be that we will not be bound linerally to time and instead experience time as God does.
The second point I wanted to make is that an inability to sin does not necessarily violate free will. Like how in the absence of a magnetic field, metal filings may orient ourselves in any direction they like, but in the presence of a powerful electromagnet their orientation becomes fixed, constant, immovable. Unless acted upon by an outside force their attraction to the magnet is inescapable. In this world that outside force is evil, another magnet attracting us in another direction. The way we choose to orient ourselves in this world determines to which magnet we become more attracted, but evil has no foothold in heaven. There is no outside force. God made certain of then by casting Lucifer and those angels who would follow him out of heaven.
Revaluation 21 (ESV) said:
27 But nothing unclean will ever enter it, nor anyone who does what is detestable or false, but only those who are written in the Lambs book of life.
It's hard to compare the choice of the fallen angels to the free will of men. Angels do not have a nature corrupted by sin as we do. They also do not live in a world where in it is necessary to rely on faith and things hoped for in order to come to God. They were with God when they made their choice, in full knowledge of his nature and the consequence if their choice. Obviously they had the choice to follow Lucifer or not to but I think many bible scholars describe this as kind of a one time or temporary free will. Once they made their choice, their eternal natures set.