vienne
The writing of letters is an expression of one's literacy skills and such can always be improved over time so its exercise can lead to improved grammar for some or many. Postings on the internet on various forums is also an expression of one's literacy and again its exercise can also lead to improvement as evidenced by p[ostings on JW Talk and the World Forum websites for JW's.
The fact that the Witness community is a publishing agency sets a good example in clear writing, easily comprehended and available to many peoples of different languages and education thus as an organization facilitates a high level of literacy including grammar.
Watchtower does not fear the informed, questioning believer as you allege but fears those who seek to undermine genuine intellectual inquiry and freedom using unquestioned, critical scholarship which lacks coherence and unity of thought.
I agree with you that finishing High School is not a guarantee of literacy and this is an indictment of education departments but such attainment can lead to improvement utilizing other pathways which not always the case. The present modern era provides the opportunity at least for advancement and this fact must be recognized and even if such ones are merely competent report or essay writers then that surely is worthy attainment for not all can be or want to be grammarians.
scholar JW