Wow, this is so fresh in my memory as it's all just happened to us.
It does sound like she's ready to go longer with her A times and you can begin to drop the cat nap. If she's fighting going down for the cat nap it could be because she probably isn't ready to sleep. I completely sympathise with the days being all screwed up, ours were EXACTLY the same while we were trying to drop it. You end up with days where you are putting them to bed at 6pm or throwing in a cat nap at 5.15pm and then they're not ready for bed til like 9pm or something ridiculous because they've recharged enough to stay awake for another 2 or 3 hours!
A couple of things that really helped to avoid really shocking days were 1) Try and stretch her morning A time to about 2.5hrs over the course of a few days. 5 min or 10 min increments each day. You're eventually aiming for 3hrs but that takes longer to achieve. It's hard, but if you can stretch out her morning a little, the rest of the day is less difficult to keep on track. And 2) Don't be afraid to put her to bed a bit earlier than usual if you need to. Basically you will need to during the transition as the stretched A times don't all happen over night. Remember she is having less day time sleep and will make up some of that over night so she probably won't wake up earlier than normal.
So if your LO wakes at 6am, try and get her through to 8.30am. This will take a few days, and obviously don't let her get so overtired that she is then unable to go to sleep. I found that when DS was flagging, I could extend him a bit by taking his nappy off and letting him have a good kick and roll around on the floor. Something about the cool air on his "bits" seemed to revive him! Some people take their kids to the store - I couldn't do this as it involved a car journey or buggy ride where DS would drop off immediately! As I say, it's not always easy, and it's not always possible, but she will eventually slide into being awake a bit longer. Just be mindful of the over tiredness thing.
If she takes a good nap of 1.5hrs or so, she should then wake up around 10am. If she naps to 2hrs, all the better. You can then try and stretch the 2nd A time - again over a few days. It'll probably be much easier to extend than the 1st A time. She should be able to go 2.5-3hrs over the course of this month quite easily. Then if she takes another good nap 1.5hrs or so, that will get you round to your afternoon A time. A lot of the girls on my birthclub have found that their LOs can stretch even further in this last A time, but again watch your DD as well as the clock.
It's a rough few weeks and each day is a bit of a mathematical beast as you're trying to get them through to a reasonable bedtime. I basically said to myself that 6pm was an acceptable bedtime and if I could make the day work out ok we would go without a cat nap. A few days we had screwy naps or short A times and I stuck a cat nap in and gave him a later bed time. You just have to go with the flow whilst also trying to stretch them without upsetting them too much.
Anyway, sorry for the long winded response but I thought I'd go through the detail. It's all totally normal, so bear with it!
xxx