Reverse capability as posted above is manufacturer specific. All of the PT6A series "could" be used as non reversing if the reversing governor and linkages specific to the reverse function were removed and a non-reversing propeller governor installed.
The only aircraft that I recall seeing that had the non-reversing propellers was a modified Beech 18 that was converted sometime in the late 1970's. The original non-reversing system was replaced with the reversing props at a later date.
Just a quick note on reversing propeller installations - there are 3 limitations on reverse power made available - the first is the internal limitations of the engine. Internal temps go up quickly in reverse and in some cases the torque limit on the gearbox can be the limiting factor - just like in forward flight.
The second limitation is structural - so engine mount - nacelle - wing structures may not be capable of absorbing the max output from the powerplant in reverse.
The 3rd limitation is for lack of a better term "Installation Geometry". The Twin Otter has this issue being a high wing with a tricycle landing gear. If not limited the powerplant could sit the aircraft on its' tail in full reverse. C of G location obviously has a role here as well.
The Beech 90 and the 99 have / had a "Beta Backup System" - they just didn't call it that - Beech term for the same system is " Secondary Low Pitch Stop System" - it works exactly the same as the Beta Backup System. I have been away from the aircraft a while but I believe there is a Service Bulletin that Beech has that allows the system to be removed - not sure if it is still on the newer aircraft so someone with some current time on the new aircraft can confirm this.
Fuel Topping Governor has 1 function - to bleed the FCU Control pressure - usually referred to as Py when it senses an overspeed condition.
The fuel topping governor is connected to the propeller reversing linkage and the speed setting is changed depending on the operational mode of the engine. In constant speed range "Alpha" the reset linkage sets the Fuel Topping to 5-6% above what the Propeller Lever has commanded from the Propeller Governor. So in this case the Fuel Topping governor will reduce fuel flow to the FCU at 105-106% Propeller speed when the propeller control is set to command 100% propeller speed. If in cruise for example the propeller control is set to 80% Np then the fuel topping governor would reduce fuel flow at 84-85% propeller speed. It is important to remember that the fuel topping governor "Trip" speed in the constant speed range is a function of the propeller lever input to the propeller governor.
The fuel topping governor is reset to 5-6% less than the selected propeller control lever command to the propeller when we have reached maximum reverse position of the Power Lever "Beta Range - Full Reverse". In this case it will limit the propeller speed to about 94-95% in full reverse - when the propeller lever is in the 100% condition. Many aircraft have an interlock system installed that will not allow you to physically move the Power Lever below the flight idle gate on landing unless certain conditions are met. One of those conditions on some aircraft is the Propeller Control must be fully forward at the 100% position.
In reverse the Fuel Topping Governor will prevent interference from the Propeller Governor. If in reverse the propeller speed should reach the 100% that the propeller governor is set for the overspeed sensing of the propeller governor speeder spring would cause the internal valve to move to block of and dump oil pressure delivered to the propeller. This would have the effect of moving the blades to a greater pitch - works perfectly when in constant speed mode - but when in reverse - movement towards a greater pitch has the opposite effect as the blade angle moves from say -14 deg to 0 deg. The result would be a severe overspeed and catastrophic failure of the engine.
Hope this helps.