Is it permissible to offer mandatory prayer while sitting?
All perfect praise be to Allah, The Lord of The Worlds, and may His Peace and Blessings be upon our Prophet Muhammad and upon all of his family and companions. Standing up is a pillar in the mandatory prayer and the prayer is not valid without it unless one cannot do it. As for Nafila (supererogatory prayer), one can perform it in the state of sitting even if he/she can stand, but his/her reward is half the reward of the person who prays while standing if he/sh has no legal excuse for such an act. And Allah Knows Best.
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.
Is it permissible to pay the Zakah to my grandmother who lives alone in a rented accommodation with bad conditions noting she had three sons.
Zakah cannot be given to ascendants (parents and grandparents). It is obligatory for her sons and grandsons to provide for her, and they would be sinful if they fail to do so. And Allah Knows Best.
Is Zakah (obligatory charity) due on the Zakah money received by a poor person, and reached a Nissab (minimum amount liable for Zakah), and a whole lunar year had lapsed over having it in his possession?
Yes, the poor who possessed a Nissab for a whole lunar year is obliged to pay the Zakah due on that money even if it was given to him as a Zakah money in the first place. And Allah Knows Best.