Hello Vicky,
i think I understand exactly what your confusion is all about.
1.What will happen to the old BDoc?
The old Bdoc will remain in error state until it is archived or set to processed state.
2. Do the initial or delta load processes the messages with new BDoc ID or through the old BDoc ID?
It will be a separate BDoc with a new BDocId. If you get an Error in SMW01 this is beyond the queueing, so later Messages will not get queued behind your faulty bdoc. They will also not "update" your BDoc in any way. Later BDocs will Bypass a bdoc in error state and update your object in CRM if the error in data has been solved in ERP before. If the data error is not corrected, you will get a second failed bdoc.
3. If the messages are processed through new BDoc id, can the old BDocs be deleted?
Best practise for the Bdoc with error would be to set it to processed state. That way there is no way that Bdoc can be processed afterwards. The BDoc will then be archived with the next run.
Best regards,
Lutz