As someone who also came to a PhD later in life (and am still a PhD student), here are my thoughts:
1. You can certainly apply - I'd focus on programmes that specifically have a joint law and economics speciality, or maybe a political economy programme. However, most programmes of which I know will still require the GRE and any other admission requirements to be met.
2. US PhDs are not part-time as far as I know. However, UK ones can be - I think Warwick also has one, but I don't know much about them.
3. You could try, but I think your chances there are <1% as they look down on people who have been away from academia for more than 2-3 years (in my experience).
4. Note that Oxford and Cambridge would require you to take the MPhil before the PhD, so the amount of time is roughly the same as in US PhDs, with obtaining a good enough mark on the MPhil serving as the qualifying "exam" for the PhD programme. The MPhil has extensive lecture requirements.