Which is better during the day in Ramadan: reciting the Quran or performing voluntary prayers?
A Muslim should establish a daily schedule for reciting the Quran in Ramadan.
Once they have completed their daily portion (known as a wird or hizb), they can engage in other acts of worship, including voluntary prayers (nawafil). Prayer itself is called "Quran", because a significant part of it involves reciting the Quran.
Allah says: "BE CONSTANT in [thy] prayer from the time when the sun has passed its zenith till the darkness of night, and [be ever mindful of its] recitation at dawn: for, behold, the recitation [of prayer] at dawn is indeed witnessed [by all that is holy]." [Al-Isra`/78]
Is the marriage contract considered valid if concluded at home by the marriage official (Ma`zon)?
Yes, it is valid as long as it is registered at the court.
What is the ruling of Islamic Law on speaking with one's fiancée (before the marriage contract) by telephone and WhatsApp?
All praise is due to Allah, and peace and blessings be upon our master, the Messenger of Allah.
Before the marriage contract, the two engaged persons remain non-mahram (strangers) to one another. Reciting al-Fātiḥah, receiving some money on account of the dowry (mahr), or accepting a gift are all merely a promise of marriage and not a valid contract under the Sharīʿah.
It is therefore forbidden for the engaged couple, before the marriage contract is concluded, to be alone together (khalwah) or to shake hands. It is likewise forbidden to speak by telephone or correspond without need. And Allah Almighty knows best.
Is it permissible for a praying person to make supplication in each Sajdah (a prostration) of obligatory prayer?
It is permissible for a praying person, whether praying individually, or in congregation, to make supplication during Sujood (prostration) in obligatory, or voluntary prayers. However, it is disliked for the Imam to do so since he is required to spare the prayer performers any sort of hardship, unless he is leading a certain group who don`t mind him extending the prayer.