Green Card Application from UK & Staying in UK

Hello,

My wife is American and she is currently in the process of applying for her UK spouse visa.

I also want to apply for my Green Card, I wanted to know the following:

1. Does she have to be in the USA to apply for me? Where do I have to be?

2. Once I receive my Green Card and I go to the USA for the first time to finalise everything, can I come back to the UK and stay here and retain my Green Card? (I know I need to apply for a re-entry visa thing to do this, but can someone please explain it for me and if there are any risks)

3. How long do I need to be a Green Card holder to apply for naturalisation and do I need to be in the USA to do that?

Thank you for reading.