Oh wow, some eerily close similarities between me and David right there.
Yeah: I have a background in computer science and economics, with a strong enough grasp of both physics and mathematics to know that David is talking bull about most of the stuff he says that uses it. In fact, I am confident enough to say that I believe David is lying about nearly all of his credentials. He may well be a Physics major in whatever school he goes too, but that doesn't mean much. It's not very hard to declare yourself a certain major.
I have combat training too. I have combat experience. I am not only an active member of the United States Army, I am an officer (1LT) in a position of authority (I am the Executive Officer of a Multi-tasked company, who's unit ID I will keep anonymous). I have full knowledge of what it takes to be a Warrior and combatant.
And it does indeed take many months, if not years, to train for it. Prior to going on a combat deployment, you will spend at least 6 months in pre-mobilization training, where you essentially spend the entire time pretending and acting as if you're on deployment (rehearsing), while taking classes and briefings about the local culture and people and the general domain knowledge relevant to the experience. You spend days and weeks rehearsing your job, what you will be doing on deployment. They want you to be as sharp and as prepared for the battlefield as possible.
David has absolutely no idea on what he's talking about in making a raw, untrained civilian who doesn't even have the mentality to be a Soldier (aside from what they grill into you at Basic Training, AIT, and your unit, it does take a certain kind of person to be able to mentally and emotionally stand the warrior's life) into a Spectre-tier Special Operations certified operator, especially with a matter of weeks or months of training on the limited, cramped deck of the Normandy, while the crew is working around with their own duties, and Shepard having to spend 9/10's of his free-time (realistically) to mission development and planning.