Sayyed Mohammad Al-Musawi

Yes of course. Lady Khadija was believing in Allah (SWT) right from the beginning of her life. That is why she was selected to be the mother of the best lady in whole human kind, Fatima (AS). Not only Lady Khadija but also all her fathers, grandfathers, Mont her and grand mothers were believers in Allah and pious persons. 
Wassalam.

There is no disbelieving child even if his parents were disbelievers. Children are born on the distinct (Fitra) which has dasics of faith. No one will be punished by a Allah, The Glorious, before knowing and insisting on denying the truth after knowing the truth. Children who die in their childhood will be given on the Day of resurrection full option to know the truth then decide. If they accept the truth, they will be in the Paradise.

Wassalam.

The shrine of Sakina is in Dariyya near Damascus. It has been reconstructed recently by Allama Sayyed Ahmad al-Wahedi  who has compiled number of narrations supporting that Sakina was a daughter of Imam Ali (AS) and Fatima (AS) including:

1. A narration in Mustadrak Safeenatul Bihar by al-Namazi, vol.5 , p.90 in which imam Zainul Abideen (AS) narrated from Sakina about the greatness of Fatimah (AS).

2. In Amaali al-Tousi a narration from Imam Husain (AS) mentioning Sakina his my sister (Wasa'il al-Shia by al-Hurr al-'Aamili , v. 14, p.168).

3. In Biharul Anwaar (v.45, p.104), a narration that Ameerul Mo'mineen (AS) called all his children by their names to say fair well to their mother Fatimah (AS) before her burial and mentioned Sakina among them. 
Wassalam.

Jinn have their own world and we are not supposed to deal with them nor are we responsible about anything in their world. Those humans who seek help from Jinn will suffer more.

Wassalam.

Quranic Du'a after Salawaat:

RABBANA HAB LANA MIN AZWAAJINA WA THURRIYATINA QURRATA A'YUN.

ربنا هب لنا ٬ترى أزواجنا وذرياتنا قرة أعين

RABBI LA TATHARNI FARDAN WA ANTA KHAIRUL WARITHEEN.

رب لا تذرني فردا وأنت ]بإي الوارثين

The marriages of the Prophets are never for sexual desires but for purpose of serving the message of Allah (SWT) including avoiding expected bigger harm on the message and testing the degree of the faith of the people especially after the Prophet's life. Prophet Noah and Prophet Lut got married with wives who were away from obeying Allah and obeying them as Allah has mentioned them in Quran as the example of non believers (Sura 66, verse 10) despite being wives of prophets. This confirms that the degree of a person is not by relation but by sincere faith and obedience to Allah and His messenger. The marriage of the Prophet (SAWA) with Aisha was because of the demand of Abu Bakr , her father, likewise the marriage with Hafsa was by a demand from her father Umar. The Prophet (SAWA) wanted to avoid the harm which can happen to the newly established Muslim society in Madinah.

The very important aim of distinguishing between real believer and superficial one, was achieved when Aisha stood and fought against Imam Ali (AS) the real successor of the Prophet (SAWA) and she was followed by those Muslims who were not concrete in their faith. Wife of Prophet Musa (AS) did similar when she led an army to fight against the successor of Musa. History repeats itself.

'Such persons attract those who carry the name and claim but don't really follow the teachings of the Prophet. Allah says in Quran: Allah will never leave the believers in the state in which you are now until He distinguishes the wicked from the good. (Sura 3, verse 179).

Wassalam.

Fasting has many benefits including what you mentioned and many more. We need to keep our aim from fasting getting the pleasure of Allah which means we fast for the sake of Allah (SWT).

Wassalam.

Yes it is permissible to buy and sell like any other lawful businesses. 
'Wassalam.

Allama Mohammad Baqir al-Majlisi is a great scholar in the Hadeeth sciences and he compiled the well known collection of Hadeeth in his famous book Biharul Anwaar. He has written others books as well. His grading is according to his own research and Ejtihaad. Other scholars have their grading which can be like or different from his grading as all the Mujahid scholars are free in their research in the narrators and narrations.

Al-Kaafi book has the most authentic collection of narrations of Hadeeth but we don't say that every narration in al-Kaafi is equally authentic. The expert scholars have their right to study and verify every narration.

'Wassalam.

Not all of the commentaries are completely authentic. Many commentaries contain unauthentic narrations or incorrect views or opinions. You need to seek guidance from sincere and knowledgeable scholars before taking any commentary fore granted. You can take the commentaries written by our big scholars according to the authentic narrations from the Prophet (SAWA) and Ahlul Bayt (AS).

Wassalam.

Allah, The Glorious created us and all all forms of life on this planet to shower on all of us His great Mercy and countless bounties which can never reach us without Him creating us.

'Wassalam,

According to the verdict of many high caliber scholars, joining the Jum'a Prayer is enough even if you are late and could not listen to the both Khutba of Jum'a.

You do not need to perform Dhuhr.

Wassalam..