I would do it and take deposit and signed contract but require earlier start. If you explain you taking big risk trusting that she really going to return work and so you require mid-April/early May start, will that help?
Locally, Sept is a horrible time to fill places. Lot of carer here still trying to find replacement for child who aged out to school. Little reply to ads.
I knew one of mine was going school and I contracted for replacement last November! Mom was still pregnant. The agreement we had was once the current child left for school, she would pay for place. She could use it FT or PT. She could take few weeks for herself before end mat leave. She could go back work early. But if she wanted space for sure, she had contract for it and pay for it. They did pay deposit in Nov 2014 and sign contract. And they did come when child aged out.
I would do this way again if could. It much less stressful knowing place full.