Whilst they're careful in the action of the film to make it clear Brian isn't Jesus, surely the central premise is that "Jesus is Brian", that his presentation as Son of God is a [comedic] mishap, or misunderstanding.
Are you trying to tell me you don't think it is saying 'people are mistaken and Jesus was just a man'? (Or perhaps 'a very naughty boy'!)
To me it seemed carefully written to maintain deniability and skirt blasphemy regulations.
FWIW it's a film I've enjoyed watching both as an agnostic and as a Christian.
And, you're right Brian is not the Messiah (which is just what the Messiah would say!).
You know Animal Farm isn't about a farmyard, right?
There are many examples in literature and film where the creators focus on a personage close to a celebrity and make them the main character. But I guess the film is well done if we can both claim our positions and believe the other person is wrong.
I don't think the writers took a post-modernist approach. Indeed, as I tried to explain, the inclusion of Jesus as a character is IMO a clear attempt to say "see, no blasphemy here" for the purposes of publishing the film. (FWIW I don't consider the film blasphemous without the Jesus scenes, not explicitly at least).
The intention is pretty clearly to say "Jesus is Brian", ie "he's elevated by the crowd to his position as Messiah, rather than that being an intrinsic part of the man known as Jesus".