Completely disagree with you on both accounts. Sorry but we will have to agree to disagree. There are some design elements of the E-wing that I like (both the one illustrate here and the version FFG chose to produce) but any way you look at it, that is a damn stupid place to put that third Laser Cannon!
I'm not sure why the third laser cannon is such a big deal. I would figure that's the perfect spot to put it: right in the centerline of the ship, but not in the way of all the avionics and sensors in the nose. The F/A-18 Hornet had its cannon in a similar spot.
It's not the centerline position that's a problem, it's the fact that it is mounted directly over the cockpit. As it stands, there is no way for the pilot to get out of the cockpit (which is O-K, as there is no way for him to get in, either ). Now, some have said that the cannon slides backwards to allow pilot access but that is just as silly, as that would 1, add unnecessary mechanical complexity to the design and 2, block access for the Astromech Droid. To solve the latter problem, would add even more unnecessary complexity. There is also the issue of muzzle flash (as if that would be a problem with real lasers but this is SW, so it matters. I believe it has even been mentioned in the source material that this was an issue, as it tended to distract/blind pilot when fired. Now, if they had mounted it under the fuselage (or in front of the cockpit), that would have negated all of the above issues. But sadly, no... that would have made sense and this is SW, after all.
All in all, Gosric's design makes much more sense is better, in my opinion.