I don't think that I was able to understand more than 50% of what you posted but here's my answer as per my guess.
You can use the main hosting domain for your main
website hosting and use the free domain for forum you wish to start. You can either put the forum in internal directory of the free domain which will be your addon domain or you can make the forum as index of addon domain.
You can swap both domains in future if you want to do so.