1. Who absorbs cost of fraud on IAP?
You do. Write code that verifies the receipt and cannot be backed-up.
2. How can we identify which users were refunded?
Refunds for consumable IAPs is very very rare. Apple understands the scam.
3. What are Apple's thresholds/limits for IAP?
Please explain your question.
4. Is Apple refunding users for Consumable products? Will they refund only last purchase or multiple purchases?
No one on this forum has ever complained about excessive refunds for consumables. There is no public data available. I suspect Apple would not grant a user a second refund for a consumable IAP - again, they understand the scam.
5. If Apple does not provide PII (personally identifiable information), what can we do to dispute or block bad actors.
You have no need to block a bad actor. Write your code to grant consumables only if the app receives a call to updatedTransactions with a valid receipt. If anyone complains tell them to contact Apple because there is nothing you can do. OR....ask them to send you their emailed confirmation billing receipt from the App Store that will indicate a purchase made by 'them'. In 10 years I have had only a few such 'complaints' and when asked to send the receipt most slither away. The few that sent me something I comped in some way. I recall one person insisting I send them a check for $0.99. I did, it was never cashed!!!!
6. Is Apple leveraging the Server to Server communication for Consumable Products (from their docs, seems to be only subscriptions). Can we be informed about refunds real-time?
No, no.
7. Whichever report we get (monthly or earlier), what is the granularity of data? do we come to know the returns by customer?
No - Apple protects its customer base. You can put something in the app to send you information when a user makes a purchase. But you can't require the user to share personal information with you. You can get a unique identifier for the user's device and for the user's Apple ID Account using identifierForVendor, writing something to the keychain, writing something to iCloud or relying on the CloudKit user record.