Ok, two things here
1) He fights humans a fair bit, Marcone and his men, evil Wizards, mortal gang members and once or twice the cops just to name a few.
2) From a mechanics argument it doesn't matter if he ever fights humans, if a Weapon: 20 make you dizzy spell is the same as a Weapon: 20 fire spell other than ascetics there's no reason to ever risk killing. Also, if the player narrates the Take Out then there is no reason he could simply narrate the Dizzy attack as lethal whenever it suited him.
1) He actually doesn't. There are literally maybe a dozen fights with humans over the course of the books (12 or so YEARS). All "fights" with Marcone are really him just intimidating people as far as I recall; often he just plans to do something like that in fact. And again, there's another way to justify why Harry does this; he's not very subtle.
2) I've already supplied a couple reasons against this here. Harry WOULD want to eliminate supernatural threats, and often doesn't have a lot of time to stick around and finish them off after "taken out" is achieved. Again, just because something is non-lethal to humans doesn't mean it would even hurt a supernatural creature. Neither of these things is a trivial point.
So Harry hardly ever fights humans (maybe once per year AT BEST), and often has to run around and kill supernatural beasts where he can't finish them off or give them time to lick their wounds. His focus on lethal magic makes a lot of sense. It makes all the more sense when you factor in his lack of subtly.
Right, and I say that the threshold between Non-Lethal and Lethal force is the gap between Weapon: 2 and Weapon: 3, and by my reading of the rules I suspect that that was the intent of the writers of the game as well.
Again, that's an arbitrary line you've decided to draw based on nothing substantial. At best you are what, basing it on the non-lethal technology we have today...which sucks? And from that you extrapolate restrictions on magic? Magic that far exceeds the standard lethal technology we have available for the best infantry? Your reasoning is not sound.
Ok, just re-read the section to make sure I hadn't missed anything, but no the number of Shifts of Power in a spell represent just that, how much power is there, in fact there is nothing in the spellcasting section discussing the relative complexity of an Evocation. The difficulty in casting a spell with a lot of shifts comes from whether you can managed to control the raw power your putting into it, not whether you are skilled enough to manage its complexity.
My extrapolation here is certainly no worse than yours. Evocation and Thaumturgy work very similarly in many ways. Complexity in Thaumaturgy and sponsored magic show power and complexity are much the same thing. That event tracks with how attacks and such work with Evocation. The more complex the spell (spray, multiple areas, etc), the more power you need for it.