1) Exactly how old is Micky?
--I think that khluva is right on this one, it's not really important... Because if you think about it... he is like 100 years old now. ha. But we do know that this takes place before KH1... so I believe that it is also before he is king.
2) How long has Micky been doing the whole Keyblade thing?
--Once again, I think khluva is right, not rellevant.
3) When did Micky get the Star Seeker?
--On this, I disagree... or more, I don't really understand khluva's answer... Think to KH2... Sora gained Star Seeker from Yen Sid when he went to him for guidance at the beginning of KH2... I believe that Yen Sid granted Star Seeker to Mickey when he was Yen Sid's apprentice. Because in KH2, it is stated that Yen Sid was Mickey's teacher.
4) Why did Micky decide to get the realm of darkness keyblade instead of just using the Star Seeker?
--On this, I definately disagree... I believe that the change of keyblade is very important... I don't know why yet, but if you think about it... Sora never actually CHANGED keyblades, just keychains, which altered the appearance of his keyblade. But the FIRST keyblade he was granted was the Kingdom Key. In the same way, I believe that Mickey was first granted the Opposite Keyblade. When or why? I don't know. But I think that when Mickey became king and was no longer Yen Sid's apprentice, he returned the keychain to him and regained his natural keyblade.
5) Did Micky know that Riku was the chosen one, but Sora recieved it instead? How much information did he know for sure.
--And I have never agreed completely with the idea that Riku was the ultimate "chosen one" and that Sora was just who the blade was thrust upon... No... The story says, as EVERYONE knows, that the keyblade wielder can either bring destruction, or revellation (this in my own words)... Why then, does it have to be that Riku is the only "chosen one"??? It contradicts with the fact that there are so many other people wielding keyblades. Mickey, Sora, Kairi, and the people in FM+SE... so with that said, I don't believe that Riku in THE "chosen one"... and thus I don't believe that Mickey purposely made himself to end up with Riku. I also think that the Kingdom Key was bound for Sora in the first place, not Riku. Riku gained his own blade (Way to Dawn)...
--Also.. on the second point... I believe that in KH, Mickey is portrayed as a normal person with normal thoughts... normal flaws... I don't think he knew ENTIRELY what was going on. He obviously knew what the stars going out meant, because I believe that something very close along those lines was taking place around the time of FM+SE... That's all I have on this question that's rellevant and I have reason for saying, moving on. haha.
6) What did he and RIku do in the Realm of Darkness besides beat the shit out of Heartless?
--I know I'm disagreeing with half of what you say, but bear with me. lol.. I don't think that khluva's answer is sufficient. In COM, when they end up in Castle Oblivion, they are out of the Realm of Darkness. I believe this because, in fact, Sora and his gang, along with the Org. were in the Castle as well... and Sora and his gang never entered the Realm of Darkness, which proves that they were indeed out at that point. Now what Mickey and Riku did whilst in the Realm, I have no clue... How they got out? I have no clue... It seems as though it would be difficult to get out if it's so difficult to get in... right? My thoughts are the we don't know everything about Mickey... in the sense that he has alot more that he is hiding... I have an entire theory behind this but currently not a whole lot to back it up, so I won't waste your time and space. lol.
7) Why is Micky able to wield Keyblades?
--I think, just like the others, A keyblade came to Mickey, just as one did Sora, Riku, and Kairi. I think, maybe, that there is a Keyblade for everyone of a strong heart, but the keyblade only presents itself in times of great peril for it's master. For Sora, it was obviously on Destiny Island... For Riku, sometime in Hollow Bastion... I don't know the actual event.. Kairi, in TWTNW when the heartless are attacking her, although, it befuddles me that instead of just appearing to her, Riku has her blade and just hands it to her, telling her to take it. This is where I get confused about that... anyways, with Mickey, it was obviously some time before FM+SE as well as the knights... I believe that there was alot of peril and disaster that lead up to the FM+SE because this battle seems alot like... a final stand type of thing... a final showdown. Anywho's.....
8) Why did Micky's ancestor(Or younger self) in Timeless River, not have the Keyblade yet, and when exactly was Timeless River? (Obviously before the Castle was built)
--Just as I said before, I don't think that his time of great peril had happened yet, thus his keyblade was still lying in waiting...
I'm sure once we figure out when the secret movie was, we'll be closer to answering these questions and more.
--And while the exact time is unsure, it is definant that it was before KH1... I believe that it is very short before Xehanort appeared before Ansem the Wise, with his memories lost and barely alive... I have a theory on this too... but that's a different story.