Thanks for answering those questions.
Looks like you have a great routine going!
OK, for the CNs then. Two ideas which might consider:
1. continue to AP the CN.
Although AP is generally not advised for sleep training your LO would most likely only habit form to AP for this one nap because she is already a good independent sleeper. The other two naps would continue to be good long naps in the cot. The CN is dropped around 6 months anyway, as it is dropped so is the AP. You might feel that this is a great chance to get in some extra cuddles and snuggles whilst she is still little and enjoy the thought of this, or you might love the idea of tacking her out in the pram or sling for a half hour walk each day and relish the chance to get some fresh air yourself, or you might feel that being bound to APing this CN is unacceptable. It's a personal decision.
or
2. As those 2 naps are going so well, you could try leaving her to sleep as long as she wants at one or both of those naps and drop the CN. You will see on the following link that some LOs have two long naps (up to 3hrs each) rather than 2 good naps and a CN. This might suit your LO and avoid the struggle with the CN, although you'd only be able to drop the CN if you got a decent extension tagged onto the other naps to make up for it.
http://babywhispererforums.com/index.php?topic=85134.0This would mean her going longer than 4 hrs between feeds and you might not be happy with that.
If her weight gain is steady and there are no other health issues I would say that she would wake if she was hungry. You might like to check with your health visitor or GP if you are concerned about her feeds though.
Unless your health professional has said that her weight is of concern I would really try not to worry about it. The centile lines show where your baby would 'stand' in a line of of 100 children. Some people are smaller, some are bigger. So long as she has steady weight gain there is no problem with being a petite person. My own DS was between the 9th and 25th centile lines for weight from birth until 12 months so he is also quite a slim chap.
Any thoughts about the CN suggestions?