Does interstellar cloud obstruct or reduce the visibility?
Yes it is possible that a cloud reduce the visibility because the matter in it will absorb some of the light which is going through. 

Yes, we cannot see all the universe and so we can see interstellar clouds up to a certain distance. Indeed, light takes time to reach us. 

If there is a star behind a cloud you will have difficulties to determine its true luminosity. In general, we define the luminosity of stars thanks to a reference star. 

I'm not sure to really understand the fourth question ...