Could you give us a little more information? When you are talking about the nose diving in 'non-planing' conditions - do you mean you are just drifting, and you have to stand right at the back of the board to keep the nose from scooping water? I find that hard to believe because the short Starboard boards have extreme rocker at the nose. Its possible you are running the mast track too far forward with your 11.8m if that is the case. But why are you sailing this in non-planing conditions anyhow? Stay home and watch tv is there is no wind to plane
Could you let us know if the lake you are sailing on is fresh or salt water? Fresh water lakes are harder to get planing on in light winds - less buoyancy. 90° is probably right for pumping onto the plane in light winds, you need to bear away excessively to get some pressure in your sails. You are a bigger guy, so for sure you might need a little more wind than others to get planing (10 knots seems reasonable with pumping).
What harness line length are you using? Boom height is best between chin and eye height I believe for most people. Any lower and you cannot put the leverage on your fin to pump on the plane and any higher and its really difficult to tack/gybe. You should be able to get out of your harness lines though! I use 26-28in lines.
Also, I don't know the Gun sails but check you are using the correct downhaul settings. Underdownhauled sails can move the draft forward a lot and might contribute to more pressure on the nose of the board - pushing it down... hard to say as I've never seen a Gun sail before.
Mast track is usually good around the middle of the track on the 161 with 12m sails and around 1-2cm back for 11m sails (depending on your sail).