Alex,
I think your suspicion is correct. Any configuration changes you make to your running EC2 instance will not automatically appear on new instances.
On to your first question:
The easiest way to test how your system works in a multi-server environment, is probably to change the parameter Minimum instance count
in the Autoscaling part of the Configuration section of the Elastic Beanstalk AWS Console. Set this parameter to, for instance, 2.
(Another way to see how new servers are started, is simply to kill your current EC2 instance. Just select the action Stop
on your running instance. The load balancer should detect this, and automatically deploy a new instance for you once the health has transitioned from GREEN to RED.)
And the second question:
Your best bet around the problem is to customize your AWS EB environment. Create a .config
file to set up required libraries and perform configuration changes to your new instances.
Good luck!