Crew Qualification impact and Promotions

On Qualifications

Hi, I'm following the LSH3 tutorial. I'm about to leave port for my second patrol (the first one after the test patrol).

(1) The green bar above the ship sections (F7 crew management), is getting bigger faster with qualified crew. But does, say for the engine room, having a full bar more impact on anything? Or is just above 50% the same effect? Does it have effect on the torpedo room(s)? What kind of effects?

(2) Also, how many helmsmen do I need and how many repair and medics do I need? It's becoming clear I need a good amount of machinists and torpedo qualified guys, so I need to minimize the others.

On Promotions

(3) In SH3Commander, I can promote my staff at will. That doesn't make sense to me. What should hold me back from promoting my staff? I'm missing something here. When is the right time and why?