In concurrence with others, one's own imagination often provides more horror for the read than if the author provides a written description--the old "fear of the unknown" concept,if you will.
I think the horror of the story is exacerbated by Mrs. White not actually managing to open the door in time. We are left to imagine what is behind the door and what precise state their son is in. Horror always works better when we are left thinking and imagining rather than when it is spelt out to us. Therefore, as I like a good fright, I think it is better that she did not open the door in time.
What good purpose would it have served for us to see a mangled, gory Herbert and to watch a mother's horrified grief? I tend to think the anticipation, the suspense, the not knowing is much more effective than whatever gruesome scene the author might have described for us. We all know what must have been behind the door, yet the not knowing is what makes the story better.
I did not want Mrs. White to open the door. This would have been a terrible conclusion to the story. Either it would have been Herbert and he would have been terribly disfigured, so where does the story go from there. On the other hand it might not have been Herbert even though the story lead us to that point, and if it wasn't Herbert, then all the suspense built up through the story would have been ruined.
I think yes , if it would have been revealed the story would have been made more interesting and the suspense would have been maintained. it would have lead the readers imagine the consequences of harbert entering the house and how the whites would have reacted. the new twist in the story would have been good.