If you list two things, it’s not a monopoly.
A duopoly can be a distinction without a difference if there's territory-dividing collusion, or even just game-theoretically "optimal" moves by the players towards a sub-optimal state for the customers.
Yep and here is a useful calculation: https://en.wikipedia.org/wiki/Herfindahl%E2%80%93Hirschman_I....
It is completely random what internet speeds you can get. The only constant is the monopoly of Comcast & AT&T.