I just love the Disney/Goff design. This decision would have been the same whether or not Verne had originally written his book.
I've got plans for a 1/48 scale ABC Nautilus version (that's going to be fun... anyone who's seen the series will know what I mean). If I ever get the time, I will attempt my own take on the Nautilus, but in the meantime, I've got three more projects waiting in the wings first.
I'm surprised that your model doesn't want to dive on it's own. I'd think that ballasted low such as yours are, they'd be even more prone to it. My model will dive when completely submerged under 3/4 throttle. If I didn't watch it closely, it could easily end up standing on its nose under eight feet of water.
I wonder what feature of the hull it is that causes the model to pull downwards? I'd think that with the upwards flare on the side keels it would tend to push UPWARDS under power...