Yeah, originally the idea of immunity was to remove the ability of someone tieing up a diplomat's time defending frivolous law suits while in a foreign country. It wasn't designed so their wife could kill some poor kid by driving on the wrong side of the road, then leave the country so they would not have to participate in the inquiry. IF she was indeed immune from persecution, then she should be involved in the inquiry, and regardless of the outcome, she would be legally protected by immunity. At least she would give the kid's parents the satisfaction of owning her guilt publicly. Also you can't apologize for something that was an accident if you don't admit your guilt, so she can't even give a heartfelt apology if it was indeed an innocent mistake on her part.
This world is full of people who will pay some great sum of money to make a valid accusation of guilt go away, rather than admit their guilt and pay nothing... It's hard for a humble person to understand, but it seems it's something every lawyer understands and is standard practice...