From your notes, can you provide me more detail on the following "convert the snapshot to image"
- How are you converting the snapshot to image?
If you have copied the snapshot from a different account, are you able to create a volume from the snapshot ? From my tests I was able to create volume from an snapshot that was created via AMI. The idea is to create a new volume from snapshot and attach the volume as a root volume (/dev/sda1)on a test instance to check if instance is able to boot. If instance boots successfully then you can create a new AMI not sure if you have taken this approach but feel free to provide more detail or correct me if I missed something.
Based on the wording of your error I could assume that you are seeing:
InvalidAMIID.Unavailable -- The specified AMI has been deregistered and is no longer available, or is not in a state from which you can launch an instance or modify attributes 
This error means that you won't be able to launch new instance from the given AMI as it was de-registered, can read more on this topic here . The docs also seem to state that "When you deregister an Amazon EBS-backed AMI, it doesn't affect the snapshot(s) that were created for the volume(s) of the instance during the AMI creation process." in practice your snapshots should still be available for use.
 Create a volume from a snapshot - https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ebs-creating-volume.html#ebs-create-volume-from-snapshot
 Client error codes for specific actions - https://docs.aws.amazon.com/AWSEC2/latest/APIReference/errors-overview.html#api-error-codes-table-client
 Deregister your AMI - https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/deregister-ami.html
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