Without an early presence of black holes, it is impossible for galaxies to have formed. No galaxies, no heavy elements near Main Sequence Stars. No such elements near stable stars, no rocky planets that have the time to develop life. No life, no observers.
So, black holes are "fundamental" in the sense that a Universe without them would be a Universe without anyone to recognize that there were no black holes!
Black Holes.
Answer
Mass that has entered a black hole isn't destroyed. Instead, it is converted into gravitational energy. Thus, no fundamental laws about the universe are violated (i.e. the law of conservation of mass).
There are already black holes within the universe
Even though black holes suck through parts of the universe, the universe is inevitably big, and growing so as the universe is being sucked into another dimension by black holes, it is also expanding.
Black Holes' can 'eat' any type of matter in the Universe.
Probably stellar mass black holes
The universe likely contains millions upon millions of black holes.
Yes.
No.
The black holes may not devour everything since the outward velocity of the matter in the universe may escapethe gravitational pull of the black holes. Stephen Hawkins reckons that even the mass in the black holes would diminish over time, though over trillions and trillions of years.
No.