I think Michonne's silence on this subject has been more than adequately explained.
In "I Ain't no Judas," Michonne tells Andrea that "I knew you were under (the Governor's) spell from the second you laid eyes on him."
Except it hasn't been explained at all... Michonne saying that is her opinion based on absolutely nothing. Andrea didn't start getting involved with the Gov until Michonne left. Andrea was initially critical and questioned almost everything he did or said, even why people call him "the Gov."
If its ZA and you find somewhere you deem as safe then someone is going to have to give you more of a reason why you need to leave other than their "feeling" or "hunch." Whether you personally would consider Woodbury a "mecca" in ZA isn't the point here. The point is Andrea thought it was safe and Michonne did not tell her the reasons she thought it wasn't. These weren't reasons right in front of Andrea's face either. Michonne went digging for them yet still kept them from Andrea.
What exactly would have happened if Michonne had told her about the blood and the bullet holes in the army vehicles?
It would have given Andrea some evidence, even if circumstantial, to doubt the Gov rather than "hunches"
Andrea would have rationalized it away or, even worse, told the governor what Michonne had said.
There has not been one time yet on the show where Andrea was given a legitimate reason against the Gov and took his side or his word over the group's or Michonne's.
Andrea demonstrated again and again and again that she is oblivious to the truth about the governor.
The Gov is very good at explaining himself yet up until the group attacked Woodbury there was never a real reason to doubt him. If she is so hypnotized then why not believe him when he said her friends had changed and are terrorists? Why go to the prison against his wishes? Why believe everyone at the prison when they finally told her the truth about the Gov?
She saw, with her own eyes the barbaric spectacle in the arena with Merle, Martinez and the walkers. She knew that Michone had sliced and diced the governor's collection of walkers (I repeat- the governor was keeping a collection of WALKERS).
Because her and Michonne didn't keep walkers too? I agree that from an outsiders perspective it seems very barbaric and improbable to accept. However, looking at it objectively the worst part about it isn't that they were fighting with walkers but that they were keeping these creatures alive that destroyed their lives and killed so many people they cared about. If you can get desensitized to walkers and being around them, it really isn't a huge leap psychologically speaking, to use them as "props" in fights since you are thinking of them as non-human objects. Of course there is the barbaric aspect of risking lives but that is apart of any style of "arena" fighting. So really the qualm Andrea should have is not with the walkers but with the fighting for sport in general. And I think she would have been upset for longer too if it was not staged.
Andrea was already confronted (many times) with ample evidence that the governor was a loon, but she ignored it.
Actually that's not true. The two times after Andrea had proof enough that the Gov was bad (before she went to the prison to confirm) both of those conversations was interrupted. She confronted him when he was first bandaging his eye but Milton interrupted as they had caught Daryl so that conversation was put on hold by those scenes. Second time she confronted him she was interrupted again by Milton that the towns people were rioting and she needed to come help. It was the third time after that when she confronted him again and then decided after that she was going to the prison herself. When was she confronted before this with ample evidence? You say yourself Michonne never told her anything.
And you're wondering why Michonne didn't tell her about Glenn and Maggie being captured or Merle trying to kill her?
Yeah, actually I am. Well not so much anymore though because Michonne explained part of the reason her going back for the Gov was to hurt Andrea. So it seems Michonne obviously did not care at that point whether Andrea believed her or not. However, later at the prison Michonne did open her mouth and say that the Gov tried to kill her first to which Andrea believed her. So I am wondering why Michonne chose to tell her at that moment when Andrea still as always only has is Michonne's against the Gov's word for evidence? Only reason I can think of is using Michonne's usual silence then to keep Andrea in Woodbury for longer rather than to do what is logical and tell (aka bad writing).
Seriously, what would it have accomplished?
A consistent and logical plot line?
p.s. its late forgive typos