So if Verizon manages to, through mergers and acquisitions and like, get a 100% market share in the US, it would still not be a monopoly, because it is not government granted?

In Verizon's specific case it would be a government granted monopoly either way. Between infrastructure easements to wireless spectrum to state laws banning municipal competition, that industry is government granted through and through.

Of course not. If you don't like paying the monopoly prices, you can just, uh... you can just start your own backyard wireless telecommunications service provider.

MCI was created to compete with Ma Bell and did so.