On the topic of down thrust, we do that mainly to reduce speed sensitivity, most noticeably when making sudden speed changes. If there were no thrust angle, the nose on a high wing plane would rise when you gun the throttle and drop when you cut it, assuming the plane was trimmed straight at mid-throttle. Yes, the wing and engine are fighting each other at full throttle, but that little bit of efficiency is a small price to pay for a well-mannered airplane.