Thank you for the wonderful questions
1. Sins are of 2 types:
1. Between you and Allah ( yourself) such as drinking unlawful substance, ignoring the calling for prayers intentionally etc
2. Between you and others
The one between you and Allah is to ask for forgiveness directly from Allah. And the one between you and others such as the example you cited in your question will be to talk to them and ask them to forgive you.
Yes some sins makes it difficult for our good deeds to ascend to the heaven. Example; usurping people's belonging, severing family relations, drinking alcohol, idolatory, fornication, injustice within marriage and so on