I am not a mullah, but few things I would like to point out.
All prophets were sinless, YES.
How literally we should understand this? They made mistakes, yes.
But in front of Allah's eyes they were "sinless". Why? Because they proved faithful, always striving for Allah's cause. They were imperfect, yet, in God's eyes they proved "sinless".
In Isaiah it says: "Even if your sins are red, they will be made white like snow - and you shall be sinless"
I am not English-speaker so I dont know how it exactly goes, but I think you can find it in your Bible.
In the bible, did Christian god approve David after making all those sins ? I think he and many are later called as "sinless" or "faithful" in the new testament.
I'm sorry, but I think we are making an elephant of a mosquito fly

And missing the original point.
One thing is for sure: if Qur'an says so, then it's correct
~Jamal