I don't understand all of your questions, but the proprietary part of the "dual license" can be anything the copyright holder wants. Qt is probably the most famous dual-license software. Check https://www.qt.io/pricing for some but not all details.

It looks like this is subscription-based, so I'm guessing you can update for the length of your subscription.

Perhaps you're asking whether you're allowed to use development patches written by external people that you'd maybe find in github discussions etc.? If you need that, you will probably have to negotiate for that option yourself. If that is your question, then that isn't really a naive question at all.

Outbound licenses (say, the GPL or proprietary) and inbound license for contributions are orthogonal. If you intend to dual license outbound, you need to clarify that in your inbound CLA.

The contributor would need to either:

- Agree to a Contributor License Agreement that grants the project owner an irrevocable world-wide commercial license (probably missing a few adjectives but you get the idea)

- Transfer the copyright

- Release to public domain