Do you believe that the Garden of Eden was located in Jackson County Missouri?
No I don't. Although there have been eyewitness reports and some statements by Church Presidents, most of what they say is unsubstantiated. The men were honest enough in explaining that Joseph Smith talked about stones as alters Adam made after expulsion from the Garden. Still, the only official and doctrinal explanations designate Adam-ondi-Ahman like Doctrine Covenants 116 as taken from Joseph Smith's journal, "the place where Adam shall come to visit his people, or the Ancient of Days shall sit, as spoken of by Daniel the prophet."
What about Doctrine Covenants 107:53-56 where Adam blesses his posterity?
It doesn't say where Adam-ondi-Ahman is located. There really isn't much of a direct line between this and Jackson County Missouri, especially in the words of Joseph Smith except second hand. When he did talk about it, the subject was almost always like History of the Church, 3:34-35 and Teachings of the Prophet Joseph Smith, p.157 with the future blessing of the Children of God as part of the last days. The geography of Adam blessing his posterity is vague in other instances.
Joseph Smith did talk about Jackson County and the Garden of Eden if you believe the reports of others as honest. Why disregard them?
I don't think the reports of what he said are wrong, but I don't know that Joseph Smith understood the revelations completely. He probably got the revelation about Adam and looked around to associate the area with the Garden. Unusual stone formations solidified his beliefs and personal speculation. It was easy enough to mistakenly match up the revelation with the place as the 1994 Ensign answer to a question on the location does. Nowhere in the Doctrine and Covenants is Jackson County Missouri described as or near the Garden of Eden except if the reader is using outside sources.
Where do you think the Garden was?
The simple answer is the pre-mortal world or in other words Heaven. Considering that all the creation narratives of Genesis, Moses, and Abraham agree with each other, I would say Adam was born or placed in Africa or the Middle East after the Fall. The rivers like Euphrates are described as located in Havilah, Ethiopia, and Assyria. Finds that contribute to Evolutionary theory agree with this set of locations (especially Ethiopia) as well. Having the Garden or Adam in America seems not very likely.
Is there any reason to continue thinking of Jackson County Missouri as or near the Garden?
Maybe to continue the spiritual tradition as a reminder of the promises God has made to the whole world. There is no reason we can't think of the Old and the New worlds as related, just like the Bible and Book of Mormon complement each other. Those who visit Jackson County can think of the Garden, Fall, and Atonement in ponderous reflection. After all, the Garden and Adam are both no longer on this Earth any more than the City of Enoch when we talk of Zion.