my son also likes to wake early.. but he goes to bed most nights like clockwork at 730. if i keep him up a little later, he will sleep a little bit later. hes a very good sleeper..
i guess what i would consider is looking for any new molars or teeth coming in. this can disrupt sleep. otherwise, you might just have to get used to the early riser. maybe try to take a nap with her during the day, or going to bed earlier. i find it refreshing to wake up earlier, even when its dark, because my day seems longer and more productive.
another thing is to try to get that nap more of a routine. im sure you do it after lunch? sometimes my son is hungry at 11, he eats then otherwise its at 12. he usually sleeps 1.5 hrs - 2 hrs. its VERY predictable. i do child care in my home, so its very routine for him to nap after lunch. if we do have to go someplace in the afternoon, i make sure that i do lunch at 11 or so to make sure he can get the nap in before we leave, or while we are leaving in the car.
the more predictible you make it the easier it should be. although, it sounds like maybe it is already predictible
i also want to say KUDOS for nursing and occasionally cosleeping! :D :D :D i think that this is a great thing to do - keeps you connected to your kid, and you are very in tune to her needs! :D this is awesome! good going mom! you wont regret it!
as far as letting her cry, yes she is beginning to understand the difference between urgent needs and simple ones... but letting her cry is not a good idea on any level. kids that age are very self centered, and they dont understand others needs. they dont understand that you want to sleep longer! theres nothing wrong with that either.. but theres nothing wrong with teaching her that you have needs. i would let her whine in bed or something, but never crying on and on.
one thing that ive done with my son is to let him have a couple books in his bed. often before he falls to sleep for nap and bedtime, he will look at his books. he loves it. i think it gives him something quiet to do alone in his bed, and he really really seems to like it! you could try that, especially in the morning, maybe let her read some books.
another thing we always do is put a cup in bed with our son. that way if he is thirsty, instead of getting upset and asking for a drink, he just gets up and gets one, puts the cup back in the corner of his crib, and goes back to sleep. :D
good luck.