Ask your HR department as it varies from company to company. At mine if you don't return you have to pay back the paid leave depending on how long you've worked there. I don't have to pay any back as I've been there 10 years. I think your paid leave will be at the full time rate but probably only if you stay on full time hours until you've served a full year full time.

Don't worry too much about the possibility of having to go part time. My first OB told me I'd probably be off work from 30 weeks due to age risk factors (I'm 42) and I worked right up to 37 weeks (desk job), I was so paranoid I wouldn't make it that far, but I did.

Most work places don't offer a second lot of paid maternity leave unless you have been back on the job for 12 months so if you fall pregnant again whilst on leave you probably wouldn't be entitled to it. Once again, check with your HR department.