1) They wanted it to get done faster. More rage = more hearts. Xemnas has a timetable to keep. It's all in his Excel sheet, down from dates, to hours, and even complex formulas involving very, very small numbers. I'm talking electron-sized numbers.

2) What, you can't take over an Organization that you were recently invited into and not have that be the great plan? Technically he didn't want to kill Sora, only have his ascend the castle, at which point, miss fake girlfriend would have completely altered his memories and made him a puppet. Everyone in the Organization knows that he's a BAMF. But little miss fake blondie screwed everything up, which caused Sora to freak out, which caused everyone else to freak out, except Axel, who seemed to be enjoying the whole process on his way to save his bestest buddy in the whole wide world.

A more serious answer may involve the importance of actually having possession of Castle Oblivion; Xigbar (and consequently Zexion) know that there's more than a few secrets in Castle Oblivion, which may or may not have anything to do with the Lingering Spirit (fine, whatever, Terra). Accessing memories of the old Xehanort could lead to untold power.

3) Ansem, Seeker of the Dark, Heartless, was simply attempting to obtain the power of Kingdom Hearts, which he believed to be darkness (an instinctive trait of any Heartless). Though the shell does appear to be so, Kingdom Hearts is actually light, so that kind of screws him over. Big giant double vision + asplosion means that you either bought Manta Style, or you just got screwed. And I don't think Ansem had a Diffusal Blade or a Vitality Booster.

Mickey probably didn't have a ship, nor a way to travel worlds, traditional or otherwise.

Leon's great at finding information, and is very strong. Who else would be better to track down, since finding the Keyblade wielder is much more difficult? Also, Mickey probably knew who Leon was based on his time spent at Hollow Bastion.

Yen Sid's his teacher, not his babysitter. A strong wizard such as Yen Sid probably isn't very concerned with the destruction of the universe.

Not their problem. Mickey didn't order them to keep tabs on Pluto. Besides, Pluto is more than capable of fending for himself, especially since he seems to have the ability to jump worlds far in advance compared to Mickey.