Sharing purchased music on multiple devices

1. You can setup Family Sharing to share purchased between the Apple IDs. click here ➜ How to download apps and content that your family members purchased - Apple Support
2. Yes. With Family Sharing purchases can shared back and forth, though only the Apple ID set as the family Organizer will be charged for the purchases regardless of where they are initiated from.
3. The same way they are now. Family Sharing does not affect devices signed in to the same Apple ID. Purchases will continue to appear there as they have without any change.
4. click here ➜ Set up Family Sharing - Apple Support

You can access purchased Music from the iTunes Store app for the other Apple ID by tapping on More..., then on Purchased, then choosing the other Apple ID from the list, then Music.

You cannot access Music obtained from the Apple Music subscription service or that was synced from a computer however.