Google IAP Billing v4 offers a Multi-quantity purchase, how do we handle this?

[FLOBUK]

Google Billing v4 is supported since Unity IAP 4.4.0, however Multi Quantity is not. See the official post:
Unity IAP package 4.12.2 is now available page-2#post-8319519

Regarding your second question, see this thread:

Setting your Google Play Public Key in Editor vs Dashboard

Hello, This message is to hopefully clear up any confusion with respect to updating your Google Play App's public key for the Unity In-App Purchasing services. Overview Since the addition of support for UGS Analytics for In-App Purchasing transaction events, it has been possible to track revenue of your app in your project’s UGS dashboard. Specifically for revenue from Google Play, our services backend can validate receipts using your app’s public key to filter out invalid receipts. Sinc…