I can’t remember if the current problem with offline installation of add-ons goes back as far as 2.1. but certainly for the snapshot the offline add-ons is broken and you should not use it.
Assuming it isn’t a problem with 2.1, one you drop the Kar file in the right folder you then have to install/activate the add-ons themselves. You can do this through addons.cfg or through the karaf console.
I know the migration tutorial talks about installing view addons.cfg as well as the console.
You can’t install snapshot bindings on 2.1 through either the console nor addons.cfg. Of you require that then you need to download each addon’s jar and install them individually by dropping them into the addons folder.
However, you are likely to run into issues if you run the newer bindings on the older the as the newer biddings are likely to depend on a change or new feature in the core and may fail to run when it isn’t there.
I don’t remember how it manages except to say that you should see errors in the log and not much will work.