(1) Tempo — that depends on the actual bar speed. In the article linked below, I explain the various possible bar speeds that could be used. But essentially, yes, if we known the bar speeds of 1–5RM efforts, then if we reach those speeds while using maximal effort to move the bar, then we know that we are at 1–5 reps before failure.
(2) Muscle damage — yes, training with a closer proximity to failure does lead to greater muscle damage than training further from failure, because peripheral fatigue mechanisms are themselves damaging. See the infographic below for more information. Technically, training with 1 or 2 reps in reserve could therefore be a useful way to train more frequently. However, we must also consider the importance of progressive overload, which requires us to push the boundaries of what we are capable of lifting from one workout to the next.
(3) Metabolic stress — yes, the processes of metabolic stress and muscle damage are actually closely linked (see the infographic above). This is why training with light loads to failure takes longer to recover from than training with heavier loads. I think it is best to train in the 5–7 rep range to maximize the number of stimulating reps while minimizing the number of junk reps and the amount of metabolic stress.
