Declage controls the AOA to which the static stability forces will be focused with no control input; in other words, the hand's free AOA/speed. A higher declage angle is the same as increasing elevator input and holding it there.
It changes the trim force balance (nose down pitching moment from the main wing, C of G relative to wing center of pressure, and other appendages that generate pitching moments, opposed by the nose up pitching moment from the tail) to occur at a higher AOA. Airplanes with moveable horizontal stabilizers, like most jets and a lot of light planes like the Cessna 180/185 family, have what is in effect, adjustable declage as their method of changing trim. What you are doing by sticking the tail surface in the different slot, they achieve by cranking a screw jack to move the front of the same surface up and down for the same result.
So the tail won't stall at the moderate angle variation there, it'll just drive the main wing to a higher AOA to achieve trim equilibrium, and your glider will trim to a slower speed. If designed properly, if you keep increasing the declage, the main wing will be driven to its stalling AOA, and the aft Center of Pressure shift that occurs at stall will cause the nose down pitching moment to overpower the tail's downward lifting force and the glider pitches over before the tail's airfoil itself stalls.
The result is, when you throw it with the tail stuck in the upper slot, it will pitch up harder for the same throwing velocity as it seeks its higher trim AOA/lower flying speed (if you throw it at exactly its trim speed, it's already in trim when you release it and will just glide straight without pitching).
So bottom line; the lower slot is a "fast slot" and the upper slot is a "slow slot". The glider will trim to a lower AOA and a higher speed on the lower slot, and vice versa on the slow slot. That's the practical result of the two options.