Installing Stream without internet?

Yes, but the free license will not be able to report telemetry so you would need a license to operate in this environment.

Yeah, I've used Stream a lot but never actually got around to install it myself.After the first startup the registration window in the GUI gave a „no internet connection error" and wouldn‘t proceed further at first.However the problem resolved itself.

You can just download the bits from https://cribl.io/download/, and install using this guide: https://docs.cribl.io/stream/getting-started-guide/#install

One more question, is it possible to use one license across multiple leader nodes?

What is the reason you need multiple leaders?