It's possible that it is Fiona, but she's just being a ween and looking for clout, honestly. That may be the most likely scenario, and why its so odd because our current picture is one where she is primarily a victim, and this may not necessarily be the case. The internet isolation/lumberjacking could be legitimate enough, but an actual wise response to the situation- go silent, stay away from the web for a few weeks, come back later, but don't dig a hole to make it worse for yourself until more of the cards are on the table.
We do have to bear in mind that the reason Bella could have turned on her could be something similar to the way she threatened Louis, as in "you fuck with me, I'll spread your bank information". It may be that, once we were onto Bella, she decided "this is Fiona's fault anyways, fuck her, I'll throw her under the bus and blame her for everything".
We do have to remember that she has a fetish for lolcows and wants to be a part of their lives, meet them, and possibly fuck them. She has also moderated the CCS subreddit, so she's not that autistic. Abnormal psychology perhaps, but not oblivious to the bigger picture. She certainly caught on to the social queues of being scared of the internet faster than Bella. It's not too much of a stretch to assume that she could want to milk the situation and center her own life as being more central to one of her favorite lolcows. We also have to remember that this seems a common enough goal with anyone involved in here, and in its own way, this does parallel wanting to be CwC's girlfriend (being the one who exposed the incest).
Now, I do not think that Fiona is worse than Bella. Bella seemed to plan a lot of CwC's last year, wanted to set them up, stole money, the whole shebang, yada yada.
What this could mean is that Fiona was more of an active participant in trying to get herself involved with CwC, perhaps for her fetish, perhaps for clout.
With the incest leak, whether this was for clout for herself, releasing it as soon as she received it was the right thing to do.
Questions which should be asked are:
Fiona knew Bella and CwC for about a month? Was she also able to view the discord server or are there messages where she saw the illicit things going on, and choose to ignore them?
Was she involved in the active plot to have CwC kill himself, in any way?
Did she know that she was potentially going to be used to blackmail CwC, after having sex with him? Was she fine with this?
How exactly was Fiona introduced to Bella?
We do strongly suspect that she released the audio earlier than Bella wanted, and presumably immediately upon receiving it. She did not send it to the police, so that should be noted, though releasing it online was largely the same thing.
This does bear possible investigation. As of now, I just see her as perhaps an ultimate ween, who got in too deep, but did do the right thing when she saw that harm was being done (regardless of the reason). I wouldn't like that to change, nor would it change how Bella is viewed, it would just make another party involved in this not as bright and shining as we had hoped.