1. Dear G, the answer to your first question is that your time spent on maternity leave will count towards the necessary five years continuous residence necessary to get UK citizenship. The answer to your second question is that the time you spent with your child will only count towards the five years continous residence if you had private health insurance whilst you were not working.
2. The need for private health insurance is that each EEA citizen living in the UK must not become a burden on the UK's NHS System. Instead they must be able to provide for their own health insurance. If you didn't have health insurance during this period, that time won't count towards the necessary five years. The requirement to work is merely that you are paying National Insurance and have health insurance as a worker.
3. Irrespective of whether you had health insurance or not, you now need to apply for a permanent Residence card. This is to show you have been living visa free for one year in the UK before you make your application for citizenship. Here is a link to the application form https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/505032/EEA_PR__03-16.pdf
4. Once you have the Permanent Residence card for one year, you can them make a citizenship application and gain Uk citizenship.
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6. If you don't want to apply for the Permanent residence card, it makes little difference. A permanent residence card is simply evidence that you have been living in the Uk permanently. However, should you wish to apply for citizenship, you will need it. My own advice would be to simply apply for it, as you will need it ultimately anyway. However, if you don't want it, there is little difference in not having it if you aren't going to apply for citizenship. As you did not have private health insurance when you were not working, this 2.5 years will not count towards a citizenship application.