Not if you use a different payment processing infra. You are not obligated to use Google's app store or their in-app purchasing system if you install the app through a different channel.

Only if it uses Google's built in payment system - which is what started all this (EPIC using their own).

Only if it's installed through the Play Store. You can IAP with your own payment platform if your app is installed from outside.

No. You are not forced to use Google to make in app purchases like Apple does. There's a choice.