What is the ruling on eating and drinking at night after making the intention? Is it necessary to renew the intention?

Eating and drinking at night, even after making the intention (for the next day), does not affect the fast, and it is not necessary to renew the intention after eating and drinking.

What is the ruling on a suitor (khāṭib) going out alone with his fiancée before the marriage contract is concluded?

It is unlawful for a suitor, prior to the marriage contract , to do anything that would be unlawful for any non-maḥram man — including touching his fiancée, being secluded with her (khalwah), or seeing her without her complete sharī‘ah-compliant covering — even after the recitation of al-Fātiḥah (This is a cultural custom to seek Allah's blessing (barakah) and mark mutual consent) and the family's approval [of the engagement]. This is because the engagement, without the formal marriage contract, does not render unlawful matters permissible. And Allah, the Exalted, knows best.

What is the ruling on fasting only on Friday, Saturday, or Sunday?

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.