Before getting into the core question here, I just want to quickly respond to the Article II challenge. Article II states that space cannot be subject to national appropriation, however Asgardia has no plans to "appropriate" or space or claim space as our territory. We will reside in space, but we will not limit the use of space by other nations. If we were to claim that the region of space between 458 miles above the surface and 460 miles above the surface as our territory we would be appropriating that space. If we simply have a structure in space placed into an orbit that does not obstruct others in space, we are not in any way appropriating space. In terms of claiming the space within a habitable structure as ours, the ISS, Skylab, and other stations have already shown that this is not in opposition to Article II. The only way this would be in conflict is if we built or positioned it in such a way as to prevent others from free access to space (such as creating a network of satellites in orbits designed to cut off any other orbit, or building a blockade to keep people from reaching the ISS or cosmic bodies). Building a station that resides in space is not in opposition to Article II, nor is claiming the space within that station as our territory. The problem would be if we tried to claim the full orbital track of that station or even the space surrounding that station as our territory. Unless we claim regions of space as ours or block others from accessing space, we are not violating this article.
Now on to the original topic. Not all micronations so fail. The Vatican, for instance is considered to be a micronation (although it has in some ways surpassed being a "micronation" given its status as an observer nation in the U.N). I think one of the key differences is why the nation was formed. Every nation, including every micronations, is formed for a reason. If ghat reason is out of necessity, however, such as needing independence from another government in order to continue one's work (the Catholic Church needing to be free from any government control for instance), or such as being in a region where there isn't strong government control (such as in space), or avoiding persecution, than those micronations carry with them a sort of heghtened sense of legitimacy based on the fact that the formation of that government was out of necessity.
More than that, how micro will this nation be? Certainly it will be small by comparison to China, Russia, or the US, but unlike many nations which number in the dozens, this nation will number in the hundreds of thousands at least. That again brings it a sense of legitimacy based on how many people recognize it as their government. A few dozen people can be written off as flakes or crazy people by those who don't want to acknowledge something. Hundreds of thousands of people are a little harder to ignore.
Finally, and I think this is the real key, there won't really be any reason to question it' legitimacy. If a large population with an organized government, a legal system, and an independent economy (as much as any economy is independent in the 21st century anyway) reside in a place not claimed by any other nation is asking to be recognized as an independent nation, who would question that, and on what grounds?
The real key of course is to establish independence in practicality (not be based within other nations without a residence beyond the claim of other nations, have our own economy that establishes trade with other nations, and so forth). Once we show that we already are independent, asking to be acknowledged as independent will come much easier. Also, once we have something to offer the nation' of the world and start developing trade agreements and treaties, nations will be recognizing us just by signing agreements with us.