I think the Top Critics on RT means that those guys are actually qualified critics and not just bloggers on some random website or news outlet with an opinion.
Yea I think the 3 movies in 2019 will be: GCS, SS2 and The Batman. Whedon probably won't have enough time to get a script ready for 2019 for Batgirl, besides all those movies were announced and further along in development than a Batgirl movie was announced with Whedon.
Maybe this director (Did a movie about surviving against sharks, The Shallows) being attached to SS 2 gives you an idea of who one of the new members of the Squad might be. King Shark?