Do words of longing and yearning between fiancés, devoid of any lustful desires, invalidate the fast?

The suitor is a stranger to his fiancée until the marriage contract is concluded. Therefore, it is not permissible for him to look at her or touch her. It is permissible to speak with her out of necessity, provided that it is not with submissive speech or words that are contrary to Islamic law, based on the Prophet's (peace and blessings be upon him) saying: "The zina of the eyes is looking, and the zina of the tongue is speaking" (the full Hadith). The safest approach is not to speak with her at all before the contract, and that any conversation be through one of her mahrams or his mahrams. As for your mere conversation with your fiancée, devoid of lust, it does not invalidate the fast, as fasting is not invalidated in this regard except by the emission of semen or pre-seminal fluid.