I'm not convinced that tangential is the correct term. Circumferential might be a better choice, but I don't really know for sure.
Not necessarily high up, so long as you can direct some of the flow towards the surface to insure thorough mixing of the wort. I have mine sort of both tangential and somewhat into the wall and angled upward at about 45 deg or so. The wort will follow the curve of the wall if you get it headed in that general direction. I'm sure your 5/8" copper tube will work OK either straight or curved, but you could probably get by with only the 90 just as well.