Giving permissions to a node to multiple accounts without creating a new role
How about using Entity Reference to designate non-owner users that may edit the node?

You could make your own module with a function to handle permission checks, i.e. "return TRUE if user is referenced on node".

Then use other hooks (hook_form_alter() for instance) to show/hide edit buttons and override edit privileges on nodes.

You can secure it by making the entity reference field only accessible by the content owner.