Dear Pareek,
1) When reversal or deletion happens automatically the quantity & value also becomes zero. Also the ROCANCEL field will be set with R.
2) Yes you are right , delivery indicator get sets when all the Grs are done. Please note that the purchase data source works based on process keys. Delivery completion gets updated only for the process key 2 (GR related data ).So only GR related key figures gets impacted with Delivery indicator. If you want the same to entire PO level then your modeling has to be done accordingly
Regards
Gajesh