You can twist and turn as much as you like, but not placing a rifle in Ruth's garage would have eliminated even the slightest possibility of jeopardising the convenient arrangement of his wife and daughters living with Ruth.
More worthless assumptions, if Ruth did in fact go through Oswald's belongings and did indeed find Oswald's rifle why would that jeopardise Marina and her children, Ruth knew that Oswald was a piece of crap and would simply ask him to remove his rifle, but to speculate that it would have a negative impact on Marina is more self serving rubbish.
JohnM