How to allow editing a single node to a single user?
I would setup all those nodes as a particular content type and author. Then you could give those users a role once they register that allows them to edit their own nodes for said content type. Then you will of course need some custom code that runs at that time to find one of those nodes and change the author to the registered user so that they can edit it. Probably could the last part with Rules but you might need to use custom php to find a node and change the author.