Hey Qwiek - you've asked me (not a HubSpot employee) a completely different question underneath my own question. This is not the best way to get any kind of information on this forum. Ideally, you would post your own question as the start of a new thread, so other people could see and answer.
Luckily for you, I do know the answer to your question, but you probably won't like it. The only way to do what you're looking for (associate a Company and Contact based on a shared Deal) would be to use a custom code action in a HubSpot Workflow. By using the Associations API, you can make this association.
However, if you don't know how to code anything yourself (Javascript or Python), this will be very difficult. Best of luck!