I am using offline as backup type , but that would only bring the DB down not SAP

Why do you need to bring whole SAP down?  If using NMSAP and brbackup to execute backup, can't see what benefit you can have there?

If you use NMSAP, there is no need for pre and post scripts - simply set offline as backup type.

As for user, you set that in nsrsapsv configuration file.

1) Give orasid sudo access to run startsap.

or

2) Modify your script to run startsap using sidadm or root.