1. The police shall make primary investigation and instantly arrest if the case of dowry death or suicide. If they are not arresting first right a letter to SP and commissioner requesting there arrest.
2. Your family members can state previous instances where husband and family demanded dowry and threatened her to kill or has beaten her or given mental cruelty, any previous instance can be cited when she told about there behaviour.
3. See any messages from sister stating the condition if any , in absence of that there can be witness who can record statement from your family, their neighbours who can state the husband and family was cruel.Any amount given to them after marriage.
4.yes it can be it shall be important document to show the amount given and there dowry demand before marriage. As dowry demand before marriage is offence too
5. The case will go on the place where incident took place the death because there is cause of action.
6. Can report.to cyber cell they can track it.
Postmortem repot it can be deduced suicide or they hanged her so it can be found out further if suicide they can held for 498a and abetment to suicide.