For ThemeForest: Basic theme features should work without any plugin. You can use a way to activate the plugin after the verification
For CodeCanyon: You will need to provide the all working structure but the activation could be use for the automatic updates and verification purposes only.
TF: I think they can create “demo” code to check the theme with the verification.
CC: You can’t lock the plugin.
No. You’re allowing the users the theme and plugins as the way they want/wish. You cannot limit the plugin usage by purchase code. The plugin should be fully functional.