2) Document number - Does SAP takes real document number? What will happen if my sales orders have different number ranges in the same plant? (ex. Range one starts from 46......01; Range two starts from 45.....01) Does that mean that my 46.....01 will be prioritized first?
The Plant is not considered in the SORT. The system will start with the documents beginning with 45. Then move upwards from there to 46. If you wanted documents with 46 to be processed first, then you should assign them a higer priority and ensure that the sort priority for delivery priority is higher than for sales document.
The reason that VBELN is used as a sort criteria is explained in the last question in FAQ note 549064.
Hope this helps.