Hello A.
Your daughter sounds exactly like my son two months ago when he was also 6 months old! Of course my son also has problems (GI problems, GERD, and major teething), but the case sounds the same!
I also tryed the Ferber method until a pedi told me that sometimes it can have the oppisite effect on a baby, I didn't believe him but the more I thought about it, it made since. The child knows that you are coming back and gets themselves more worked up.
He told me to just be totally consistant with the bed time routine, bath, books, bottle, music (I chose to use the same CD) then lay him down, pat his head, tell him that I love him SOOO much and close the door. The first few nights where rough, crying (like yours) anywhere from 15min-2hours (of course I mad sure he was ok by peeking in). I tried to make myself busy too, so I didn't have to think about why he was crying, like doing the dishes, folding clothes, picking up. After 3 nights exactly, he started to fall asleep without even a wimper, he did talk to himself, but real softly.
We are still having other issues staying asleep, but at least it's not a fight to get him down anymore, that really was soooo stressful, so I know what you are going through with that.
Good luck and I hope I've helped a little.
Z.
p.s. I also had the same nap problems, and when we got him falling asleep at night, down, he started to take 3 naps a day, for a total of 4-5hr, I think that he was just over tired all those days and that too made him not what to sleep at nap and bedtime, and he his a much better baby than the one I'm used to!!!