I suppose the method Lisa taught us was designed so that each new topic creates a new discussion, kind of what we have here on Discourse but not what you expect. You want something like Messenger where there is only one 1v1 discussion between two users, right?
In that case, I think we can use some logic to hide the “Sending a chat form” button when a discussion has been kicked off. An idea is to have a template column joining the email of signed-in user and the second user, then have the same template column in the “Topics” sheet. Make a relation column matching them, if the relation is not empty then don’t show the button.