This is a very beautiful and logical interface, but it's the wrong approach, because it's a very developer-centric approach to writing. The problem with writing is you can't loop through it and find whether each sentence passes or throws an exception. A written work needs to be evaluated as a cohesive whole. That's what "bold and clear" writing means to me: a written piece of work that stands on its own and says what…
Eh, this isn't so infeasible. Any wide-coverage, statistical grammar will have a TOP --> NP production, as well as the usual TOP --> S. i.e. the grammar must accept NP-only sentences; they're too common to reject as mistakes.
I think it's reasonable to have a linter that throws a warning for that sort of sentence though. By all means dismiss them. But it can help you catch things you didn't intend.