Define a dev environment with a single instance and a 'load test' environment that has settings close to production environment

You cannot have multiple development environments in Elastic Beanstalk, just one development and one production environment

Create an Application Load Balancer to route based on hostname so you can pass on parameters to the development Elastic Beanstalk environment. Create a file in .ebextensions/ to know how to handle the traffic coming from the ALB

Use only one Beanstalk environment and perform configuration changes using an Ansible script