(1) yes, muscle protein synthesis (MPS) rates very often return to baseline long before the muscle is repaired. Typically, MPS rates return to baseline within 24 hours in trained individuals, but muscle repair can take anything up to 4 weeks if the workout is severe enough.
(2) I’ve debated Stu both here and on Twitter about motor unit recruitment, and the last time we discussed it, I think I managed to get my point across. Motor unit recruitment is maximal during ballistic exercise under non-fatiguing conditions, which causes no muscle growth, so it cannot be the sole factor. My Mechanical Loading article explains in more detail.
Mechanical loading and *not* motor unit recruitment is the key to muscle growth
You have probably come across people who argue that reaching full “motor unit recruitment” is the key to muscle growth.
(3) yes, that is pretty much exactly my view
(5, sic) I am not sure about the role of training status on muscle damage.
Clearly the repeated bout effect means that the first workout of a series always causes more muscle damage than subsequent bouts. But this doesn’t differentiate between intermediate and advanced athletes.
The amount of fascicle elongation during a muscular contraction is the main cause of muscle damage, and it differs between regions of a muscle, depending on the exercise. IMO, this is why the repeated bout effect happens, and why we get sore after incorporating a new exercise in our program, even when training the same muscle (obviously there is overlap between exercises, too). So people who regularly perform a lot of varied exercises will hypothetically be less prone to damage than those who don’t (since genetics is probably not the factor).