Hi Aditya ,
From my point of view you have to achieve this with the help of Generic extractor using FM.
1. Your document numbers are already there in your custom tables now you need to check for some field in this table only to identify the document number getting changed/created during that period.
2. Once you get these document numbers you can select the relevant data from BSEG/BKPF in the select statement in the Open cursor statement in the FM Source code .
Also please refer http://scn.sap.com/message/14307484 .
This is relevant and you also have some code for your reference.
Your other 2 approaches doesn't look feasible to me .
Thanks