While AWS ES can be cheaper in some configurations, Elastic Cloud is actually quite competitive in pricing for larger clusters when compared to AWS' ES-service. This post compares the two services, and there's an example price comparison at the end of the post: https://www.elastic.co/blog/hosted-elasticsearch-services-ro...
We support most official plugins, and if you get a gold or platinum subscription you can upload your own plugins. Elastic's X-Pack is included in every cluster, which includes security features like role based access control.
It's not possible for external service providers to integrate with IAM at this point.