Q: Valid English sentence (grammatically) or not: Has that we have arrived back at our starting point proved that the world is round?

A: unacceptable

Q: Valid English sentence (grammatically) or not: This is a problem that you'll be able to tell the folks up at corporate headquarters to buzz off if you solve it.

A: acceptable

Q: Valid English sentence (grammatically) or not: John talked to a woman who came up to him.

A:
acceptable