Is it permissible to perform Hajj on behalf of a deceased fiancé, and what is the proper intention and required actions if that is permissible, knowing that I have already performed my obligatory Hajj?

It is permissible for the fiancée to perform Hajj on behalf of her deceased fiancé, as it is not a condition for the proxy to be a mahram (definition). It is sufficient for her to enter into ihram (definition) from the nearest miqat (definition) to her, intending Hajj on his behalf by saying: "I have entered into ihram for Hajj on behalf of so-and-so, and here I am, O Allah, for a Hajj on behalf of so-and-so." It is preferable to vocalize the intention.