Well, she was innocent on this on, but after all what she did, she was in a really weak position. In doubt and with all the evidence pointing at her, she had it coming. Though MC should have investigated it a bit further. Well, I guess she thought a bit about it, before deciding to fire her. We only saw the lay off talk.
On the matter of the other two maids: That long haired one is stupid. She knows how bad her own standing is. That mistake (of somebody else) could cost her her job and worse, it might have cost her her own health or even life. And judging from the head maids previous behavior, that personal maid should easily arrive at the conclusion, that this was a calculated move and that she herself would also have been sacrificed, as the head maid didn't gave her any warning. There is no reason to keep quiet or even talk with the head maid afterwards.
About firing all 3 personal maids. She couldn't without management rights and if she would have asked the head butler, he would have talked with all persons involved, who in turn would have denied everything. Nothing would have happened, but it would have shown the maids, that MC touch was still limited. Keeping them in terror was much better until she won the rights back.
And I guess, it all amounts to, that her image IS really important. At first I also thought, why didn't she just tell the head butler, that arsehat, about the sabotage? But this was right before he transferred the rights back to her. There would have been a chance, he would have reconsidered it. But this way he didn't even know something went wrong, as the kitchen maids didn't tell anyone about it and she bought the goods and compensated with her own (secret) money. Also this way next to nobody knows about her secret funds. Keeping up the image was a wise move.
The only thing I'm pondering about is, why the head maid kept her mouth shut. It was her plan. I guess she wanted to wait until things blow up, but afterwards, why did she bring that "blunder" to light? She is the head maid. One inspection of the kitchen at the right timing, she would have "noticed" that the wrong things have been delivered and could have told that the butler. Even if MC did manage to pull a saving move, it would have come tumbling down. How did she manage to buy the needed commodities? Did she sell something? At that point her secret funds wouldn't have been secret anymore. (Though MC's excellent deal might have boosted her reputation further.) Also her inability to create a grocery shopping list would have been shown, which was her goal from the beginning.
And as we have shown, that her image might be from really big importance, we see why she didn't fire the 3 maids as soon as she had the rights back. She "forgave" them, before she had the authority to do so. Turning back on that would damage her reputation again. But we also saw, that as soon as there is a slight reason or doubt, they are gone. Given a few months MC can also simple fire the left two maids without any big reason. Those two maids should see to create as much space between themselves and her as possible, as then, just maybe, they might be spared, as they might not be that important/pose any danger to MC any more.
@Xeredge: Nah. I believe she welcomed the chance to get rid of a backstabbing bitch. You shouldn't forget, that MC can't trust that maid anymore anyways. Innocent or not, it's a headache less.
@WhimsiCat: Hrmrmrm....well, but only to raise her reputation further. Though I would keep her away myself and the silver in the future. Personal maid? Not going to happen anymore.