Your statement about shortening their lives is the first I have ever seen. When I first saw ROTK I dived deep into the meaning of Frodo leaving andf possibilities of him being there for Sam...The main view from Tolkien's writings that were relayed from people was that time was different there. It wasn't known how Frodo might age compared to Sam in that time. Mostly that Frodo would not age as quickly as in mortal years. Its all debatable. :)