After I had prayed, I found a drop of menstrual blood and was unsure when that happened; what is the ruling on my prayer and fast?
All praise is due to Allah, peace and blessing be upon the Messenger of Allah.
Your prayer and fast are correct ,and you should not make up for them, and we ask Allah, The Almighty, to duely reward you for these acts of worship. Your menses start once you see blood because the situation remains as such unless there is certainty or predominant conjecture, as per the Fiqh maxim: "Certainty is not removed by doubt". And Allah Knows Best.
Is Zakah (obligatory charity) due on articles of merchandise even if they were stacked in the merchant`s stores for years?
Yes, Zakah is due on goods even if they were stacked in merchant`s stores for years, in this manner Islam struggle against monopoly.
Is it permissible for a Muslim to slaughter an Aqeeqah on behalf of someone else, and offer it to him as a gift?
In principle, the guardian is the one who should offer the Aqeeqah (the sheep slaughtered on the seventh day from the child`s birth) because he is obliged to provide for the newborn, and it is impermissible for anyone else to slaughter it on his behalf unless with his consent. However, it is permissible for a person to offer the sheep, or its price as a gift to the guardian of the newborn, and then the latter can slaughter it, or deputies someone else to do that on his behalf.
Is it enough to say Tasbeeh (Saying Subhaana Rabbiya Al-‘Atheem during Rukoo`, and saying Subhaana Rabbiya Al-A‘laa during Sujood ) once, or twice during Rukoo`(Bowing in prayer) and Sujood (Prostration)?
All perfect praise be to Allah, The Lord of The Worlds Yes, saying Tasbeeh once during Rukoo` and Sujood is enough, provided that the worshiper`s attentiveness of the heart isn`t undermined during each. As regards the recompense, it depends on the number of Tasbeeh. And Allah Knows Best.