I don't know anything about ELB but in a typical load-balanced situation every Moodle will (and must) have the *same* wwwroot setting (mymoodle-aws-dns.com in your case). The mapping between that hostname and the actual physical server is done by the load balancer. In other words, it's nothing to do with Moodle. Moodle doesn't know its running in a load balancer. This should be in the ELB documentation somewhere.
Installing and upgrading help
wwwroot and AWS Elastic Load Ballancer
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