Am contemplating setting up a forum which has a single category and forum node. Is it possible to have the forums tab point to this single forum node and have it basically 'invisible' to the user so what is now the forum list would just be a view of the threads within the single forum node? The breadcrumb would just show Home > Forums
You can change the Index Page Route to make the landing/index page the forum.
You can either use a Route Filter or an .htaccess redirect to automatically forward forums/ to forums/forum.
The only way to modify the breadcrumb would likely be by editing the PAGE_CONTAINER template (it would show Home > Forums by default but you don't want the Forums element, although it wouldn't matter with the redirect in place).