Sounds like your ex may be unhappy that you've moved on - it's not necessarily that she wants you back, it may be that she just can't sit with the idea that you are happy with anyone else. Human nature, I'm afraid.
As for the rest, I'm afraid this is going to sound rather harsh, but the idea is that you get it right and things get better. Basically (I have custody of my girls) my ex cancelled/missed contact on a regular basis and I did exactly what your ex is doing, which was to stop contact while I went back to court to get the contact order varied for my youngest - I had contact changed from twice per month to once per month, and only refrained from asking for it to be stopped altogether, or down to every 3 months because my daughter still wanted some contact, and her wishes were paramount.
To go back, the original contact order was twice per month, but almost immediately she started missing contacts with flimsy excuses at short notice, or no excuses at all. The effect this had on my daughter was very bad, and I was the one who had to pick up the pieces every time. Since the variation, over a year ago, my ex has missed a couple of contacts (two in a row in fact, so 3 months between contact visits) and my daughter now simply expects that contact could be cancelled at any time - it upsets her when it is missed, but it no longer surprises her.
This is exactly what you have to avoid, and I'm afraid you aren't doing well. Whatever the reasons, and yours sound genuine, your daughter simply knows that you aren't there, and I can guarantee that your ex won't be sticking up for you. So you need to ask if you can start contact again, and, short of death, you make absolutely sure that you make it. If she won't agree , then try mediation. Also, in the meantime, phone her at least weekly - it may not be a specacular conversation at her age, but it's contact. Also, don't forget that whn your new baby comes along, Lucy is going to feel left out of your attentions, the new baby gets you 24 hours a day, Lucy won't, so you've got to make exra effort.
As for maintenance, I can see exactly where you are with the provision you are making, but if your ex goes to the CSA, the simpe fact is that if you are earning more than £100 per week, the CSA are going to take 15% (12.75% after your new baby is born) as maintenance for Lucy and much as you may not trust your ex, she does have to pay for food, clothes , houshoild expenses etc to look after Lucy.
As I said, this may sound a little harsh, but my intention is to try to get you on the right track to having as much contact and as good a relationship as possible with Lucy.