What is the best way to extend/override something in the suitescript 2.0 files? For example, I need to customize stuff for TransactionHistory or Subscriptions but both are 2.0 and they are not included in the ssp_libraries.js file and so it doesn't seem like I have access to them from within ExtensionDevTools. Do customizations like this need to be done within the SC_Live folder?

Yes, you will need to use the SCA developer tools

@Steve Goldberg, following up on Bryce’s question. Can you extend suitescript 2.0 files like we do with traditional SCA client and server side script (prototype method replacement, wrap, etc? Or do you need to override them?