First of all, I'm bothered by Aizen's insane All Accordingto Plan because I said so as much as the next guy (I think it would had been better if he was just good at Xanatos Speed Chess) but villains aren't known for growing, unless is a Face Heel Turn.

His death started all the conspiracy theories (in SS only a captain can kill another). Part of Aizen's plan was to make distractions (intruders, fighting among captains, etc.) so Unohana didn't prioritized Aizen, since he was already dead and given her healing above almost all else (even enemies) credo.

What exactly made Gin so great, at least in that perspective, they're different types of villains to begin with, Gin is an Obviously Evil and Faux Affably Evil while Aizen is more of a Maipulative Bastard.

With a 400 word limit there's only so much detail I can put in. For now I'll focus on the Unohana issue.

Unohana was not confused or distracted as to what was going on. She explicitly states that she knew when examining Aizen's body that it was fake and that Aizen was using central forty six as his hideout the entire time. Armed with the knowledge of who was likely behind the conspiracy and where he was Unohana proceeded to do absolutely nothing for days. She went to the execution just to watch, not even to try to stop it like Ukitake and Kyoraku, and then ordered her lieutenant to go after Renji to get Rukia back. She waited so long to do or say anything that what she knew would have been useless if not for Ichigo. What good would knowing Aizen was behind it all do once Rukia was killed?

And as I said, it effectively ruins Toshiro's role. We've watched him build his suspiscions, fight Gin, be framed, and now decide to use his own method to stop the execution (the time it takes him to get to central forty six is another problem. No other captain travels that slow ever, including Toshiro. By all rights his confrontation with Aizen should have been many hours before the execution, but Kubo couldn't make his 'reveal' before the fight between Ichigo and Byakuya). But all of that plot and buildup we've had with Toshiro is rendered moot because the captain we've seen the absolute least of so far shows up and announces that she's figured it all out days ago. I stand by my statement that it can be considered a deus ex machina.

As for Aizen, he did not have to do anything he did in that arc because his hypnosis is so broken. There was no sign that Hitsugaya would have suspected anything if not for his overhearing his staged conversation with Gin, so we are left to wonder what the point of leading him on was. With his illusions Aizen could have easily fooled everyone into thinking he was just doing his job as he conducted his research and never given anyone a hint that there was a conspiracy and that the execution might be dangerous. Then, instead of seeming to want to kill Toshiro, which would have made the whole game he played with him make some sense, Aizen seemed to want to get out of central forty six before Toshiro got back, hence his question to Gin as to Toshiro's arrival while they were leaving. If Aizen's powers were less broken this could be explained away, like if he could only fool one person at a time and was using it to make Unohana believe the faux body had no flaws and that when he attacked Hitsugaya with it his hold over her faded and she noticed that it was just a fake body.

When I say he didn;t gro beyond that, I mean his actions never made aby more sense than the above.

Soi-fon was the one who ordered all lieutenants to go after them, and Isane even looks at Unohana upon this, as hinting that she knew her captain wasn't really confortable with this, but perhaps to avoid more unneeded conflict between the same group, told her to go along.

Who says that wasn't planned, just like almost everything else, after all the plan only works if he suspects Gin specifically, and planting that suspicion would had been the best start up

His plan included made as many people as possible distrust each other, even people with close relations were on opposing sides (Kyoraky/Ukitake and Yamamoto, Renji and Byakura, Momo and Hitsugaia, etc.) his death made it "almost" absolute that no one would suspect him (since pretty much anyone could be seen as a suspect) on anything or at least of what was happening after his apparent death, after all it's later revealed that he is the only person left related in some way to the only other incident of betrayal 100 years ago.

What gain could he had by killing Toshiro? to me he was just part of plan to, in Aizen's words "spread out the enemies". He just deduced that he was going to central 46, he killed Momo because even Hitsugaia was impressed of what she had done so she was a lose thread.

Aizen's power are very broken but even so to an extent. He wasn't expecting Toshiro to come back so soon, so he saw them, if he was he could had just made another illusion and escape without being noticed (not that it mattered by that point). Also your reasoning would only make sense if Unohana inspected the body again, since her senses were deceived, it wasn't that false information was planted in her head like Tsukishima's ability.

But yeah, I get your point. That was a big Idiot Ball on Unohana's part. Though I never understood the tittle. No one said that Bleach was perfect back then, I thought you were going to talk about current complains that were present all the way back in the SS arc.

2deep4u?

But seriously, very little of Aizen's plan would have worked if he HADN'T faked his own death. Even assuming that his ability is as flawless as people think, which it really isn't, there are a lot of things that only his death would have accomplished. Getting the Captains to fight amongst each other. Causing people to become suspicious of Gin. Keeping anyone from trying to visit the Central 46 to appeal the execution. Observing Ichigo, which we later found out was part of his experiments. It might have meant that everyone would have been around when he ripped the Hogyoku out of Rukia's chest, which they're going to notice, especially since he can't hide his Reitsu and Ichigo can see him.