These things are inherent complexities in any situation with backup/DR. You've already got to figure this out. When one machine is degraded and another is trying to recover from db logs, that's a cluster.

> This decision should be made per-project

This is one of the least project-dependent things ever, akin to source control. If you don't have scripted installs you don't know what you're running and you can't reliably test it or upgrade it.

You don't need to run a cluster, or have auto-scaling groups, just because you can.

Depends on who manages the fleet. An EC2 instance provisioned by Elastic Beanstalk is quite easy, the problem is EBS can be an utter PITA of edge cases and weird failure modes.