Talking to one's fiancée in Ramadan with romantic talk may lead to arousal, and if pre-ejaculate fluid (Madhy) is discharged as a result, it does not break the fast. However, it is advised to avoid actions that provoke desire and lead to this outcome. If semen (Mani) is discharged as a result, it does break the fast.
The discharge of Madhy doesn't invalidate one's fasting but it invalidates the ablution and his clothes must be washed because they are impure in this case. As for Mani, it breaks one's fast and making up invalidataed is obligatory, beside; a Muslim suitor should abstain from these matters during Ramadan and not to degrade the holy month to this extent. And Allah Knows Best.
Do the Jinn have anything to do with the disappearance of objects?
Jinn are one of Allah creatures, and whosoever lost anything should look for it and never accuse niether the Jinn nor humans of stealing it. And Allah Knows Best.
Is it permissible for a person, who vowed to offer two Rak`ahs (units of prayer) everyday for Allah`s sake, to do so after the Fajr (dawn) prayer?
In principle, a vow should be fulfilled as it was originally intended, and if its time lapses then, it should be fulfilled at any time because of the vow. Therefore, it is permissible to offer them after the Fajr prayer, and there is no prohibition in doing so. However, it is preferable for him to offer these two Rak`ahs at some other time to avoid the disagreement amongst the scholars in this regard.
How should I deal with the whispers of the devil with regard to ablution and purity?
Whispers in ablution are from the devil, and so you should take refuge in Allah from the devil`s evil suggestions that aim to distort the religion. Thus, once an organ is completely pure, don`t question its purity since having whispers doesn`t mean that the purity is invalid as it has already been established.