To answer your question about heat, when the reverse spike is present, the diode routes it back to its source. The coil will dissipate the spike and heat up ever so slightly, then cool down very quickly. Some heat is also generated in the junction of the diode, because the semiconductor junction presents a voltage drop, 0.7v is typical (why they call it SEMIconductor), so there is some resistance there. But it happens so quickly that the heat doesn't have much opportunity to degrade the component.
However, because heat vs. time is the life-limiting factor on any semiconductor, it will degrade over time. This is usually the reason your perfectly running machine suddenly smokes a diode or transistor, with no obvious cause. Silicon junction degrades, voltage drop increases, resistance increases, joint heats excessively, melts down, then fails.
If the diode were a switch contact with close to zero resistance, there would be no heat at its junction, and instead, the coil would absorb the spike completely. And the switch would live forever.
So, if you're replacing a coil, replace the diode as well.