The node level autoscaling doesn't need to be integrated with kubernetes, all it needs to do is create a new instance and register it as a node through normal channels.

Even if you don't need autoscaling, I'd suggest still using autoscaling groups and setting it to a fixed number of instances, so that instances will automatically get restarted if they go down.

Yeah, any new machine instance has to join the Swarm (and its equivalent in kubernetes-speak). But that can be decoupled from kubernetes or docker swarm mode.

As for image management, it would depend on how you would like to propagate new images. With a private docker registry, you could potentially point each new instance to the registry and take care of propagating new images. I favor this approach since it keeps everything separate and easier to manage.