The correct form is Chris's. The after the <'> is only omitted when the noun is plural ending with . The <'s> should be kept in all the rest of the cases.
In all of your examples, the reader will understand what you mean in number.
Focusing "newspaper reader's thumbs" versus "newspaper readers' thumbs", notice that when you generalize the plural form is preferred in this case (it's not only one reader but many).