Tara, agree with Myia - start with 2.5easy and then work your way up. I don't think you should topping her up though, it sounds like she is getting enough from the BF (since you mentioned she is not really interested in it). Maybe instead of top up formula, you could express some milk and give her top up EBM (might not seem ideal considering you are up at night & having hard time getting her to sleep). Its just that formula can be difficult on some babies and I do think that upping up the concentration of the formula would have caused her discomfort. I was told to put rice cereal into Isabella's feeds when she wasn't sleeping all night (around 3 months as well) & I did! She was extremely constipated and I felt really really st*pid that I even listened to this person and cause discomfort to my lo. Believe me, when you are so sleep deprived you will want to try just about anything to get them sleeping.
About EASY - it doesn't always fit into the "ideal". My lo was not on true EASY until the 45-min naps stopped, which was somewhere around 5 months. Honestly, you can only do so much when they are taking 45 mins naps. I did what Jazmyn did - if she was awake before her next E, then I gave her some A time and watch for tired signs. If she showed tired signs around the time she was ready for a feed, then feed her and put to bed. As long as she doesn't fall asleep at the breast or bottle, its ok.
I would definitely not cut naps before bedtime - or have her awake for long time before bedtime. Try not to have her awake any more than 1 1/2 hrs (which is the ideal for her age) but if she has only taken a 45min nap and you haven't been able to extend it, then don't keep her awake for the full A time. The last A time is generally shorter as well so by cutting out the last nap and keeping her awake for a long time will cause even more disturbed sleep.
I know how hard all this is for you, dd2 was a terrible sleeper. Up all night and had really inconsistent naps during the day. Sometimes 45mins, sometimes 2hrs. Take heart, this will pass and it will get easier.
Take care of youself
Layla