Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.
Here are the answers to your questions in their respective order:
1- No. Major sins can only be forgiven through proper repentance. For repentance to be valid, one must fulfill certain conditions: If the sins involve the rights of Allah, here are the requirements to be fulfilled: 1. Feeling deep remorse over the sins; 2. Refraining from them immediately; 3. Forming a firm resolution or intention not to do them. On top of these, one should also do whatever good deeds such as charities and the practice of dhikr and du’a and reading the Qur’an.
As for the sins involving the rights of humans, one ought to fulfill another most essential condition: seeking absolution by compensating the victims or getting the forgiveness from them. If it cannot be done as they are dead or not found, then one should still seek compensation by praying for them and giving charities on their behalf, etc.
In conclusion, making istighfaar and reading the last verses of Al-Baqarah simply do not absolve a person of the guilt of committing major sins.
2- Making lots of istighfaar and reading these verses and others – followed with reflection and meditation — would help nurture the mindfulness of Allah and hence would restrain a person from committing sins.
3 and 4, please see the answer to the first question.
Almighty Allah knows best.
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