And the shahada too..
Some of them (the commoners not the clerics/ulema/mullah) even laughed when I told them what would Muhammad's Shahada..sounds like:
"I bear witness that I AM a prophet of God"..
Usually it did started some electricity inside their brain to start thinking...
Salam / Peace
That's what I tried to explain to my father few days ago. In vain.
I asked him why the Sunni shahada was not in the Quran. I told him that as Abraham, Moses, Jesus and other Prophets were all muslims (before Muhammad), it'd be illogical that their shahada was the Sunni one. I also told him to add Muhammad in the Shahada was making a distinction between the Prophets. He agreed with me BUT as Muhammad was the one who brought the final message, it was normal to add him - according to him.

And also, another explanation he gave me is that it was a way to differentiate Muslims from Christians and Jews. I don't see any reason to do this because as 3:64 says:
Say: O people of the Book,
let us come to a common understanding between us and between you; that we serve none except God, and that we do not set up anything with Him, and that none of us takes each other as patrons besides God. If they turn away, then say: Bear witness that we have submitted.
How can you come to a common understanding when you absolutely wish to differentiate from them on the basis of making a distinction between the messengers?
And eventually, he told me that Islam as a whole does not only come from the Quran, but also the hadiths. And the usual, you don't know how to pray without hadiths, you don't know how to do wudu without hadiths, blah blah. For the wudu, I proved him wrong and showed him the verse. It didn't change anything. I didn't tell him I don't believe in hadiths anymore. Questioning the Sunni shahada is enough for the moment, I think.
Well, I cannot give guidance. But I'll definitely try again.
Peace