I see your question as boiling down to the following: There is no sin in heaven, but free will continues; is it possible for God to create such a world via fiat? If yes, why did he create the garden and not heaven? I will suggest 2 possible answers consistent with versions of libertarian free will. The answers are complementary.
- God can create such a world by fiat, and this is it.
- God cannot create such a world because it would be internally inconsistent.
Answer 1: God can (and did create such a world)
Libertarian free will in its most basic sense is the ability to make real either/or choices. If, when you wake up in the morning you are genuinely free to choose to brush your teeth or not to brush your teeth, you have libertarian free will. This type of free will does not imply total freedom, since you are not free to brush an alien's teeth even if your choose to do so. We are, however, able to genuinely choose to do or not to do what is logically possible.
Furthermore, most advocates of libertarian free-will do not require all choices to be either/or, so in this sense you still may have genuine freedom not to brush your teeth, but by consistently choosing to brush your teeth every morning for 10 years, brushing your teeth ceases to be a choice and instead becomes an involuntary behavior.
To expand on this sense of "habituation" consider the process of learning to read. A young child begins by memorizing the shapes of letters, and the basic sounds that those letters symbolize. In time, children learn to put the sounds together in the same way that the letters are joined on a page in the form of words. With even more time, reading no longer consists of sounds but words and eventually phrases.
Similarly, the choices we make form habits that accumulate into character so that much of our behavior and personality is an expression of natural (virtually involuntary) impulses.
Walking, riding a bike, talking, all move from conscious, focused choices into unconscious higher-order behavior. These are still the result of genuinely libertarian choices, but they accumulate into a mature you that does not choose things that are out of character because of the accumulated free choices that they made before. In this way, advocates of libertarian free will can hold to both its reality, and to its formation of a person into something that is not arbitrary but entirely consistent with themselves.
Through this process of maturation, a person can therefore be transformed from an arbitrarily free child to an entirely consistent adult. This is the exact process that the Holy Spirit kicks off in a new believer when they are born again. Go enters the heart of a repentant sinner and restores their dead (enslaved) heart and will to live (freedom). Out of this freedom, believers are now capable of making choices that accumulate into new habits and entirely eliminating old ones (e.g., alcoholism and pornography can be eliminated from a person's life, while hospitality can be habituated to the level of a reflex).
These are examples of how God can habituate us into the type of humans who do not sin, and the connection between libertarian free will and non-sinning is the process of habituation. This process continues until death, at which point our resurrection bodies shed the last influences of the flesh and we are totally habituated to sinlessness. It is in this sense that it is possible for us to be possessors of libertarian free will, and yet never sin in heaven.
So far I have been attempting to explain how there can be both genuine libertarian free will, and yet sinlessness in heaven. Once we understand how such an outcome can be achieved, we realize that it includes the process of birth and maturation which include making choices that define us more and more. God did create such a world by his fiat, and we live in it right now. The externalities of such a world include many people whose free choices at a young age (many of them influenced by parents/society) accumulate into total resistance to God, the gospel, and sinlessness.
What I am suggesting is that to fiat heaven as you describe it, with genuine libertarian freedom, entails fiating a world that includes lifetimes of choices that constitute the history of the human race. The garden is a world with libertarian free will without the accumulation of character, whereas heaven is a place full of those who, having libertarian free will through the regeneration of the Holy Spirit, have developed the character of sinlessness in imitation of Christ. In other words, the creation of heaven (as you describe it) entails the creation of the garden.
Answer 2: God cannot logically create such a heaven without such an earth
That is how I understand free will, the garden, and heaven. At this point, however, you may believe I have misunderstood your question. If you are questioning whether God could have instantaneously created heaven full of people who are both genuinely free and yet entirely sinless, then I will have to answer no, given the nature of libertarian freedom. Only libertarian freedom combined with the accumulation of choices over time is compatible with sinlessness, and such a timeline virtually necessitates some people making choices that are corrupting, not sanctifying.
For people to be both free in an undeveloped libertarian sense (which is essentially capable of choosing good or evil arbitrarily) and for all of them to never sin is not logically possible. More importantly for your question, even if they never sinned, the process of training their free will to the point where they would never sin (not could) had to occur across time.
In this sense, God could not create that kind of world by fiat. To create such a world would require a compatibilist sense of free will such as is espoused by Calvinists and is ultimately a form of determinism. My answer here gives a bit more background on the importance of responsibility in free will, and why I think God chose libertarian free will.