A1: 1. If your situation is such that having a second business in no way contravenes your intention to continue your green card job, you are fine using it to get to do another job or to have your own business. I would advise you to talk to your own lawyers.
2. Whatever questions they ask it is your right to do so and the only focus of the inquiry USCIS should take is whether or not you have the intention to continue working with your own employer; the one who filed your adjustment of status and that should be their focus of inquiry. It shouldn't matter to them whether you have a second job, one side business or ten.
3. You should be able to file the I-485 again. More...
You can try. 214(b) denials are difficult to overcome and highly unpredictable.