[motto]

The premise for my statement is simple.If fired in a vacuum a projectile will only have two forces acting on it, gyroscopic and gravity. It will fall to earth with its axis still pointing in exactly the same direction it left the barrel. The only force that makes a projectile change its axial orientation to align with the direction of travel is aerodynamic.
In the case of vertical firing this aerodynamic force doesn't come in to play as it is square on the nose. Comes the point the projectile stops climbing but is still spinning at hundreds possibly thousands of RPM.
Falling backwards, it depends on factors such as shape, centre of gravity and RPM remaining as to what happens. If gyroscopic effect is preponderant, the projectile will fall base first. If aerodynamics are stronger, the projectile may turn over or tumble. This could happen at any point during its descent.
I would be very dubious of the outcome at this angle and at what point away from the vertical aerodynamics have the preponderance to ensure arrival nose first is any body's guess.
In regards to mortars, the type I am familiar with are smooth bore and the bombs have straight fins. There is nothing to impart spin. I have vague recollections of mortars with rifling but, if the bombs for these have fins, the spin may be for the prevention of wobble and the fins to keep the bomb pointed in the direction of travel.
I know nothing of howitzer shell construction but maybe they are fused in such a way that attitude on impact doesn't matter or maybe it explains why a significant number were reported to have failed to detonate.
If any body disagrees with any of the foregoing, please speak up as I welcome discussion and am here to learn.
David.