While adding or editing the code of the plugin goes a bit outside the range of support that we provide on this forum, I'd be happy to provide my recommendations to get you pointed in the right direction.
If you're looking to only edit the content of the messages that you describe above (i.e. change the submit success text to a unique custom message), you can do this by editing the .PO file in your plugin's PO directory. You can use a program like POedit to do just that, then generate a new .MO file, and upload both into your "po" directory. And this will allow you to set your own custom messages. This page walks through it as well:
If you're looking to implement more advanced jQuery functionality into the plugin's processes, your best bet would be to take a look at the plugin code to see what can be added or worked around the current existing code. I'm afraid we don't have any documentation that directly addresses adding custom handlers, but it's something that I can pass along to our developers to provide support for in a future release. The plugin code can be viewed by going to WP Admin > Plugins > Editor > MailChimp.
If you have any further questions, let us know!