I don't buy any studies that only measure muscle protein synthesis. They are maybe kind of okay for making a hypothesis, but they are kind of useless for the whole getting fitter thing. The theory is nice, but those ideas rarely are capable of making predictions (like nitrogen balance and testosterone/cortisol ratio.
I linked to a study where they actually measured strength, which showed the opposite of their conclusion. For now, I still believe the side with evidence.
I actually think that the idea of muscle protein synthesis was either made or popularized by the supplement industry. Now the supplements don't even need to build muscle, they just have to increase muscle protein synthesis (it increases pretty much every time you eat protein, by the way).
As for your last question, based on what I've seen, it looks like the answer is yes. The study on rugby players (if they were in fact elite, like he claimed) then their gains were truly truly impressive. The studies have tested the effectiveness of training with as little as 20% 1RM, as well as walking, and I don't think there is a lower limit. The first time I tried it, I did curls with a 500 mL water bottle, and I could feel the burn (whether or not that would produce results, I don't know, but I can guess...). I kind of wish there were more (or any) long term studies on it, but I guess it's too taboo.