There are no minimum income requirements. I got approved in Taipei with very low income as they would not accept a new contract as I had not been paid from it. They accepted a couple of other part time contracts I had. They want to see income paid into bank accounts.
15 business days for approval after submission. You can be asked to submit more docs but the website does not give reset time, so it could be 15 more days or less.
Border run is the simplest way to get the new 180 day stamp. Can extend via an immigration office which requires evidence of the 500k seasoned for one month along with your DTV docs and other immigration forms.