Hi there 
Please have a look at Can I share my Forge Account? which I believe will address some of your questions.
From what I’ve read, we can get a single account and assign all of our Foundry licenses to it, so that many games with different people could be run simultaneously.
Different games can be run from the same Forge account provided that you are a player or GM in those different games, and they aren’t being ran at the same time.
And can multiple people create worlds on that account, or only the person with credentials to that Forge account?
The person who owns the account can create new worlds, then invite others and assign them the GM role, letting them run it.
How is Forge assigning licenses to the world, if multiple are available?
The premium content linked to the top/first Foundry license key in the list of license keys is accessible.
Hope that helps!