How will a prolonged series of steep turns produce a stall in subsequent level flight?
A prolonged series of steep turns will not produce a stall in subsequent straight and level flight.


  "after perhaps twenty turns have been completed, it will stall: stall, mark you, out of level flight with cruising throttle!"


In this case "level flight" means not climbing or descending while still in a steep turn. 

Stopping the turn by rolling level would unload the wings and prevent the stall. Nosing down would also unload the wings and increase airspeed, also preventing a stall.