Fiona Poisoned Sunny?

[Foxy]

The first time I read TGG as a teenager, I thought Fiona was pretty despicable for abandoning the Baudelaires for Count Olaf and breaking Klaus's heart.  However, in my most recent readings of TGG, I can understand her vantage point.  She had only known the Baudelaires for a few days, and her brother was the only family she had left.  Plus, she and the hook handed man ended up double crossing Count Olaf anyway, which maybe Fiona had planned on doing all along, so really I don't think she was bad at all.

But...

In TGG, Fiona is the one who helps Sunny into her helmet when they are leaving the grotto.  Is there any possibility whatsoever that Fiona put the MM spore in Sunny's helmet?  And then, she didn't want Violet and Klaus to help her research the antidote.  Did she want Sunny dead?  Why?

[Dante]

I'm not sure this follows. There's no trace of a motive; it's unbelievable that Fiona would want to murder Sunny and Sunny specifically. Fiona pursues the Medusoid Mycelium specifically for the promise of converting it to medicinal use; I think if she had wanted to poison anyone out of pure scientific curiosity, she'd have done it to herself, after having thoroughly researched an antidote and prepared one in readiness.

I revisited the section where Fiona helps Sunny into her diving helmet, or rather proposes to do so, and it's not clear at what stage Fiona is in relation to re-equipping her own diving suit. I would suggest it would be very difficult for Fiona to have contaminated Sunny or her diving helmet without risk to herself unless Fiona were already wearing her own diving helmet; since the narrative does not specify this, it's unlikely that there was any foul play. Additionally, you have to remember that we're talking about miniscule, invisible spores here. It's incredible to suppose that Fiona would be able to pluck one from the air and move it about.

Fiona's behaviour in the immediate aftermath of the discovery of Sunny's poisoning is influenced by the inexplicable loss of her one remaining family member; she doesn't rely on Violet and Klaus to help her research at that time because the person she most relied upon in the world has just abandoned her without explanation, and so in her mind the only one she can rely upon is herself. At the same time, I wouldn't underestimate the significance of her yielding control of the submarine to the Baudelaires; she's effectively placing the search for her stepfather in their hands, while devoting herself to the urgent question of Sunny's health. If she simply didn't care about Sunny's welfare, it makes more sense to me that she would have done exactly the opposite; taking the submarine's controls herself, and sending Violet and Klaus to research something out of their area of expertise.

In any case, Fiona plays a vital role in persuading Fernald to betray Count Olaf and help them to return to the Queequeg so Sunny can be cured, which would be completely counter-productive for any murder plot. She even lets them escape when Carmelita catches her! This entirely collapses any supposition that Fiona can have had an interest in Sunny's death.

I think this theory could only work if you believed that everything Fiona ever said about herself was a lie. If that's the case, where's your evidence that she lied; and where's your evidence for what her true personality is?