They needed the axe whether she'd resisted the turning or not surely? It took a while for her blood to 'infect' the Vampires didn't it? It's the edges again, a literal sharp edge.
Nanny accidentally triggered how to resolve it but Granny can't always know if she's 'right' or not, like with the child murderer. She makes decisions but she can't know for sure they're always the right one which is why she's always interrogating herself on which way to go and doing that for the people who can't make that type of decision. You need to understand the 'grey' more than the black and white and see the least bad and most good to weigh your decision. The child murderer wasn't 'all bad' and some people can remember the man who was OK because they didn't know what he was capable of. Death shows up at the still birth where the midwife doesn't know what to do - the child is already dead but is the mother going to die as well? Who has Death come for? Just the baby or the mother/wife as well? Will the husband be left with no one? Granny doesn't know that does she?
So even Granny is plagued with doubts, but she knows she IS good at making decisions and trusting herself. She saw a way to pull defeat out of the inevitability zone and a way to neutralise the Count's advantage because she did
know that he would want her cowed and under his influence. Therefore she opted for the not 'million to one' chance but a gamble
be a victory if she could pull it off. She chose that because it was the only choice she had - she knew they would
be defeated if she didn't do it, so no option but to do it, knowing that she could still be defeated. Either way Death showing up for her wasn't
an option because she wouldn't die whatever happened. THAT
was the certainty. But she would possibly have been a vampire so she didn't know for sure, until her blood began to take effect on them in Escrow and when Agnes realised the meaning of her not being turned by Vlad.
From that point yes, Granny knows she's got them for the moment at least because her blood has weakened them so much - however she's also at the very limits of her own strength in keeping her borrowing on top of their nature because it can only be kept going so long before she'll get like Esk when she borrowed the eagle for too long...? The natural form reasserts itself eventually and will take the borrower with it - so she's actually still in danger of being turned
if the others don't take advantage of the temporary effects of her borrowing whilst she can still stay in control of the Vampires. So even at the castle there was still a chance of the Vampires getting their usual act together again...
So no, I don't think she knew she would win because she couldn't keep on winning indefinitely. All she was doing was buying the others enough time by staying on top just long enough to let all the others see to exploiting the temporary
weakness she imposed on the Vampires. It's always far braver to fight when you're not sure of victory and this is why Granny has to be hard on herself and her instincts, because if she thinks she's always going to win then that's actually a weakness isn't it? To be truly victorious you have to be scared of losing everything, else you don't try hard enough - and you can then lose your own 'edge'...