Re: newbb 4.0
honestly i didnt take a single look to SEO neither used them anywhere.
IMO Core team should solve SEO in core for the whole site otherwise you have some links from some modules different from others.
But Im pretty sure you dont need those user-friendly 301 redirect URLs because when you enable the SEO option, old URLS will be redirected. please try yourself.
If you have any issue with SEO it would be better to ask from Alfred.
Also IMO Your first problem was because You could not find that config in the first look and it is because the preferences of newbb is too confusing.
I plan to simplify them and unify all configs in plugin.php and preferences.