Bonus points if the submarine isn't chibified and the robot mode isn't stretched out/lanky or sporting humungous curves of kibble everywhere.

I figure: split the rear of the sub and stick it alongside the lower legs like Springer's tailboom. Have the robot hands raised over the head to form the front of the sub, with the forward hull splitting and retracting over the forearms like Lugnut's cockpit (but pegged, not springloaded). Fold out the front halves of the sub to thicken up the sides of the chest like a vertical version of G1 Magnus's chest hinges. Collapse the torso into the chest a bit. Do another Lugnut-chest on the shells surronding the upper legs, retracting them down to the outside of the lower legs. Use double joints at the hips to allow the tops of the upper legs to rotate slightly up and outwards to form a wider stance and reduce the robot height. Split the chest and move each piece outwards. I'd suggest folding the conning tower into the torso behind the panel that hides the storage hold for the head.

This should allow for a robot mode which is nicely chunky, has very little kibble sticking out, and has full articulation as well as fists and feet which don't have to be part of the hull proper.