You can have NRO and NRE at the same time.
There is no time limit to keep NRO account open before you transfer from NRO to NRE account. The transfer can be done on the basis of certificate from a CA in Form No. 15CB. The CA will have to certify that the taxes as applicable are paid to effect the transfer.
The amount from NRE can be transferred to your US Bank accounts without any problem.