Offhand, you're asking too many questions in one post. You'll find that if you ask focused questions you'll get more responses.
That said, a matte white screen will provide a slight to moderate improvment in PQ over your typical white wall. If for no other reason than the screen, with it's masking, will provide better contrast for the image. There are many other smaller factors that a screen can help overcome in a wall: incosistent color, uneven surface, etc.
Other screens will provide other benefits. Read up on these forums and many of your questions will be answered.
That said, a matte white screen will provide a slight to moderate improvment in PQ over your typical white wall. If for no other reason than the screen, with it's masking, will provide better contrast for the image. There are many other smaller factors that a screen can help overcome in a wall: incosistent color, uneven surface, etc.
Other screens will provide other benefits. Read up on these forums and many of your questions will be answered.