Autoscaling is your friend. If you're not leveraging it (multiple availability zones), you're doing it wrong. Even single instances can be launched in autoscaling groups with a desired capacity of 1 to ensure that if it falls over, a new one is spun up.
Point 2: AWS is likely trying to rotate capacity for updates, which means they need to evict instances. That are running on doms that they need to update/deprecate/etc. The longer your instances are running (or the more specialized the type of instance is), the more likely you'll see an eviction notice. It should be part of a good practice to launch new instances often as new AMIs become available, or as private AMIs are updated for security patches, etc. - at least monthly! Autoscaling and solid config management simplifies this practice greatly.
Good Luck!