What forces act on cascade-type thrust reverser blocker doors?
ATTENTION, very coarse analysis !

This is a bit vague, but maybe this will give you a starting point. 

Generally on turbofans, about 70% of the total thrust is provided by the fan.
If you consider a high power brake using 80% of the max thrust, this gives you 
TADAAAAA 25000*0.8*0.7 = 14000 lbs (yup that's rocket science).

Now, this gives an approximation of the power provided by the fan. However, the outlet angle is not parallel to the main flow. Assuming that the mass flow rate as well as the magnitudes of the velocities are conserved in the transfer and that the outlet angle is about 45° from the engine axis, it gives

qm x (v2-v1) = F where qm is the mass flow rate, v2 the outlet velocity vector and v1 the inlet velocity vector, while F is the force of the fluid on the structure.

Because of the angles this gives qm x (0.5+1) x v1 = FX and qm x 0.5 x v = FY on the doors.

Then, assuming (yes, I said to pay attention at the beginning of my answer) that Ffan = qm*V1 = 14000 lbs, which is quite doubtful unfortunately, we obtain FX = 21000 lbs and FY = 7000 lbs.

Regarding stresses, it is IMHO far to dependent on the actual parts of the engine to assess.

If anyone can check this to be sure, be my guest ;).