I'm totally not following this at all haha.
What do you mean better result? I think you're comparing the Tamron 28-75 f/2.8 with the Nikon 50mm /f1.8 AF-D right? You also know that the 50mm outperforms nikon's profession 24-70?
I think you mean sharpness and focus? Do note that when u r shooting in low light and at f/1.8 on ur 50mm, you have very little depth of field. Means that anything that is not in ur plane of focus will become blurry. I guess thats where ur comparison comes. If u stop down ur 50mm to f/2.8, no way it's worse than ur tamron, unless u have a bad copy of the lens.
Is there any way to select more than 1 focus area? I think it's automatically set by your camera, it either selects the face or whatever it "thinks" your subject is, then measures all other points in the picture which lines in the plane of ur subject and thus gives more than 1 point highlighted in the viewfinder.
U can't focus on items that are in different planes, u can only work with your DoF by making it very deep. (unless u have your focus at infinity like in a UWA) So lets say u have 2 people standing 5 meters apart, one in front, one behind, unless u stand very far away, it will be hard to have both of them in focus.
Do i use autofocus much? Usually, but very often i find myself changing to MF while shooting cause the camera keeps locking on to the wrong thing... ... Hmm actually i use MF more than AF. HAHAHAHA. woops.