The Conservatives' EU referendum bill could fail in Parliament.
It's not THE Conservatives' Bill, it's A Conservative's private members bill which is why it may fail, and if it does the chances of Cameron winning in 2015 increase big time as he can paint labour and the Libs into a corner. Cameron is prevented from making it a government bill with government time by the terms of the coalition agreement. He has given it his full support.
So to your question
"do you think that (in the unlikely event) if Cameron gets re-elected in 2015 he will truly offer a free and fair referendum on the Brexit in 2017?" then yes I believe he will make it a manifesto and if he wins (unlikely but possible) he would make it a Queen's speech commitment asap. I trust his integrity but not always his judgment.
You suggest he wouldn't because he wants us to stay in the EU. The evidence is he does not as the EU stands so where he campaigns will come down to how much change he can get. We just don't know, he likely does not know either. What would you expect him to do if come the day he believes it is best for the UK to stay in?