Is there an engine thrust level limit for deploying thrust reversers?
Engines must be at idle thrust to enter reverse thrust.  This may not be 100% true of the physical action of engine entering reverse thrust (opening the buckets or translating the cowling), but it is required by the thrust levers.  In all airplanes I'm familiar with the thrust levers must be at idle to enter the reverse regime,  In the EMB-145 the reversage was below idle (i.e. you keep moving the levers aft) through a gate.  Even if the engine is capable of entering reverse thrust not at idle, there is no way to command it without first being at idle.