Hmm interesting.. you know, I'd completely forgotten about engine placement >.<
I like your suggestion for having heavy guns instead of dual light guns - it would certainly bring something to the table that no other ship currently does (other than, as you mentioned, the Galleon). I was a little worried my suggestion was too close to the Mobula, this would certainly mix things up a bit.
I also like your suggestion for the helm placement.. I'd actually drawn something like that myself, but I erased it since I couldn't figure out where to include the helm on Riggatto's version. It basically makes the corridor layout look a bit like a
peace sign ^^
I basically agree with everything you've said (including the balloon/hull hardpoint placements) with the exception of two things - First, I believe the rear guns could both be located on the upper deck on either side of the main engine, forcing crew members to switch decks if they want to cover the back of the ship.
However, rather than joining these decks with ladders, a ramp in the middle could lead from the lower front of the ship to the upper back, with passing spaces on either side (or the reverse, with small ramps on either side and a corridor down the middle). Possibly there could be ladders on either side of the helm cabin leading down to forward guns, which would also allow a shortcut for engineers looking to get to the balloon hardpoint as fast as possible.
It would work a bit like the Mobula does is some respects.. there would be two areas to be divided up between two engineers, as well as additional guns for them to use when they aren't repairing. If anything it might be more complicated for the gunner, as he would have to run back and forth a fair bit if the engineers are busy (or if the pilot is asleep and doesn't think to turn the ship fast enough xD)
The other thing was the shape of the ship you outlined.. I realize it's only a basic text layout to give us an idea of what you were talking about, but I actually envisioned the hull and balloon being integrated together much like Mobula.. a sort of.. hamburger shape, a bit like this:
______
/ ____ \
_/_/ H \_\_ Front View
\__G___G__/
\______/
______
/ ____ \
_/_/L M L\_\_ Rear View
\__T___T__/
\______/
As you said, its main weakness would be poor visibility and a massive target in the shape of the balloon.. however by sandwiching the hull inside the balloon, it might help split the damage between hull and balloon, and make it a bit more challenging to take down - especially by ramming.
It would also have something of a blind spot above and below as well, since the turrets are all relatively close to the hull, there wouldn't be any way to look directly underneath or above - a small ship could conceivably hover there while they recover from an attack.
I'm still kind of keen on the idea of the crows nest coming out of the top though.. it would negate the visibility penalty somewhat, but as I mentioned it would take crew members away from the main hull, making it more or less unusable in battle. It would be useful for spotting at a distance though, and a well coordinated crew might even make use of it tactically (for example, outfitting Field Guns on the rear, and relaying directions to the pilot to fly backwards while they get in some long range hits).
It probably isn't necessary though.. I guess it would be down to the folks at Muse to determine if it added or unbalanced anything.