You should read the "Memoirs of Ijon Tichy" by Stanislaw Lem (author of Solaris), where all the problems you were talking about were discussed and a reasonable explanation for posibility of time travel without the creation of alternative time lines is given.
Slashdot Mirror
"Memoirs of Ijon Tichy" is not even a quarter serios, but is extremely entertaining. you might have a glimps at the first page on amazon at http://www.amazon.com/gp/reader/0156586355/ref=sib _fs_top/102-9350581-1008139?_encoding=UTF8&p=S00E& checkSum=2y6cDzBjNqfywNgJ4sQPrTywFr5E9VKc3f%2FF305 %2F6aE%3D#reader-link
You should read the "Memoirs of Ijon Tichy" by Stanislaw Lem (author of Solaris), where all the problems you were talking about were discussed and a reasonable explanation for posibility of time travel without the creation of alternative time lines is given.