In a sense you've answered your own question. Shaw, as you know, loved to write lenghty explanatory prefaces to his plays, explaining his philosophy, what he viewed as the social significance of his work, and so on. But in as much as these prefaces were often written well after-the-fact, arguably they are of no more importance than the analysis of any other critic. In other words, Shaw's word is not necessarily the last word.
Morevoer, 100 years of productions of his plays have shown that these wonderful prefaces add nothing to the actual performance. Shaw was a brilliant dramatist, and a brilliant critic, but dramatic criticism doesn't play on stage! Shaw himself is reported to have told an actress, "Never mind what the speech means, my dear. You read it beautifully."
So - I suggest you take the view that the preface to "Mrs. Warren's Profession" adds an interesting perspective on the play, but you have to take Shaw's comments on his own work with a grain - or two - of salt.
I hope this is helpful.