That's great that she went to sleep in her cot! She'll probably do a short nap but that's OK. I would sneak back into her room when she has been asleep about 25 mins, and watch her, and when she starts to stir see if you can gently shh-pat her so that she stays asleep. If she wakes up fully you could shush-pat and/or PUPD for 20 mins to see if she will go back down.
She fed at 8? Regardless what time she wakes up, I would try to hold off feeding her until 11, or as close to 11 as you can. Remember, she is not to fall asleep on the breast, so if you think she's going to nod off, take her off the breast and do a quick diaper change or something before putting her down again. She has to go into her crib awake.
If she has a short nap, keep her next awake time (including the time you spend trying to get her back to sleep) to about 1.5hrs max, so that will mean starting to get ready for her next nap after an hour or so. If she has a longer nap, you could start wind down after 1.5-1.75hrs, to have her asleep by 2 hrs after waking.
REgarding changing one thing at a time or all at once - I guess it depends on the temperament of the baby and of the mother! Me personally, I would change everything. I figure, everything is going to be chaos for a few days anyway, might as well get everything right how I want it - short term pain, long term gain.
Other mums who aren't as stubborn as me, or who have babies who REALLY don't cope well with change, might tackle one thing at a time. I never had the patience for that.
Besides, to my mind, it's difficult to change one thing at a time. I mean, eating obviously affects activity and sleep, and sleep affects eating and activity - it's hard to change things in isolation! Like with your little one, it would be hard to get her sleeping well if she's not eating well, but also, if she's overtired she's going to want to feed for comfort, so it goes both ways.