That's true - but taken in context he might have just been trying to impress on Dresden how dangerous she could be so he wouldnt underestimate her.
I dont know if Mavra is any stronger than any other BCV.
So she certainly could be real BC - but I tend to think not. She doesnt seem to have quite the same level of power or knowledge of someone like Cowl or Ferrofax [who I think are BC definitely. Plus in DB she was working against the interests of Cowl, which works if you think she's an agent who can be used by BC to stir up trouble, without her being aware of all of their plans/plots.
Would not that first part mean that she should indeed not be underestimated
No sadly Jim hasnt given us anyone else in the court to compare her to
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.
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?
Wouldnt that mean that A) Mavra is a Backstabbing BC member B) someone one the outside who trying to screw stop them, or C) shes a BC member taking orders from the higher ups which even Cowl doesnt know of hence Cowl too is a Pawn who does not know anything or D) Cowl is not actually BC and Mavra was sent to stop them in their interests?