Urgent query - RCM when both buyer and seller are Unregistered

I am a provider of works contract Service. I am still unregistered as my turnover as of now is Nil. I got a contract and I will receive my first advance payment from my customer after 5 days of Rs. 19 lacs. Still I am not required to get registered under GST.

After 15 days at the time of making my first payment to a registered supplier, I will take GST registration voluntarily. From this day onwards, I will get ITC on all the inward supplies.

1. If I receive any inward supply before my registration, am I eligible for the ITC on it?

2. And if I receive any goods/services from an unregistered supplier before my registration, Am I supposed to pay under reverse charge subsequently?

Please give detailed guidance on the same.