If the intent is to understand a crime, you don't start by making assumptions about who is guilty. You follow the facts where they lead.
The Saudis may or may not be involved. We know they cut people up using bone saws. So their assurances aren't something anyone should be considering.
Clearly, Trump is beholden to the Saudis as he is to Putin. That is the only safe assumption at this point. His own interests are already having an impact on the investigation.