Would not that first part mean that she should indeed not be underestimated
What I meant was it was more like he's saying - Be very careful - dont get careless or drop your guard kind of thing. At least that was the way I took it.
No sadly Jim hasnt given us anyone else in the court to compare her to
Im hoping we might see some more in Book 15.
Mavra's been pretty quite about her power level. Remember how in BR Eb thought she just had a little skill with dark magic? she trained a small timer like Bianca to face off a wizard like Harry with some scary spells which probably refect on some of the stuff she knows but hasnt been seen doing.
Also Ferro is in the same league as Mab (maybe even a little stronger) so unless Cowl or Mavra pull of a power ritual theres no comparing either of them to someone like Ferro. Based on the power thats been demonstrated by the two, we cant really make a judgment one whos stronger at the moment.
See, my impression based on what we've seen so far is Cowl is stronger than Mavra. His powers seem far vaster - he put up a much bigger fight the times we've seen him than Mavra did in the homeless shelter. Yes, she got away, but my impression of Cowl is he wouldnt have run that quickly. For instance, in WN, instead of retreating when Dresden was getting the upper hand, he called in the ghouls. But this is just my impression.
Im not exactly sure what you mean in the last part, if Cowls BC and Mavra were an agent to stir up trouble then why does it make sense that the BC would send her to mess up his trip to Godhood?
Sorry I didnt explain that well - let me try again. I agree with the op who said they see the actual BC members as a small tight-knit group who tend to use agents to accomplish their aims. These agents probably know only what they need to and not the Big Picture - and may be total dupes [I have the feeling the Red King was egged on to prepare for war against the wizards but never really told the truth about the BC or their ultimate aims.].
The way I see Mavra [based on what Ive read so far] she is more another pawn/dupe/catspaw they can use as needed. I think they tell her as much or as little as they need to in order to get her to act the way they want her to act. IOW she is being manipulated by them. But that doesnt mean she is ALWAYS working for them, or has any idea of the big picture. She's not in their confidence to any great extent. And when they dont have a "job" for her, she's on her own.
I think in DB she was on her own and wanted the book for herself. My point was if she was BC herself, she would have realized Cowl was getting the book for them and wouldnt have risked involving Dresden.
But like I said - Im not entirely sure about Mavra. The only 2 I think are BC for sure are Cowl and Ferrofax.