Alphas have nothing to do with it I believe.
An update edge error, if I am not mistaken, means that you have 2 planes that are on top of each other and share the same vertices. It's a mesh error not a texture error. The only thing you need to know about texture errors really is in compiling the textures themselves and making sure that the unwraps are good and that you don't have any spikes in the unwrap. (spikes being where 3 points in an unwrap of a plane are perfectly in line)
Your error is a geometry error. Whatever you are doing with alphas is not causing it. They are simply a detail and/or reflection mask.

To fix this error of the mesh however, you need to import the .wrl file that was created when you ran tool and got the error and look for the red colored planes.

oh!
Well that's simple, the vent must be labeled as 2 sided property. shader_name% Then it will not give the error. Also, note that the alpha on the bump (normal) bitmap is what defines the transparency in this case, not the one on the main bitmap. Not sure if you knew that, but this will allow you to have both a specular mask (alpha of main bitmap) as well as a alpha for transparency (bump map alpha).

If you do not want/need it to be two-sided, then all faces behind (or otherwise inaccessible by the player) must be ID'ed to materials with '!' in their material name. The '!' denotes non-collision geometry, and as long as your vent remains collidable, you will not have any issues. In other words, you're making all faces inaccessible by the player non-collidable so you do not get update edge errors.