Too many pieces of evidence point directly to Saudi Arabia, not excluding the fact that the murder was apparently committed inside the Saudi consulate in Istanbul!
The only cover story I can think of is for MBS to report after a "thorough investigation" that one of his close aides was on the payroll of Iran.
That when the aide was confronted he committed "suicide." Then show pictures of the basement of the aide's house with stacks of US dollars, gold coins and Iranian riyals there.
I admit this is weak but it is probably the best that can be done. The 15-man hit team will surely be willing to state that they took orders from the aide, thinking it was an order coming from MBS. That the aide misled them.
The best twist to this story is bringing Iran into the picture. It continues to paint Iran as evil and explains why the Khashoggi killing has so many Saudi fingerprints on it, the Iranians wanted it that way---to break up the US-Saudi-Israeli axis to bring "peace" to the region!. MBS can then state the axis will not be weakened by this devious Iranian plot.
This sort of gets the focus off of MBS and of all people the poor saps in Iran.