2. 8 months, yes if they are taking interview abroad. 4-5 months if they are already in the U.S. and applying here. It depends on how quick you are in responding to the government's request for documents.
3. You have to file separate petition for each of Mom and Dad. Expenses wil be around $1500 (attorney's fees are not included) per person.
4. You can bring unrelated translater at a Green Card interview.
5. Citizenship is 5 years to go after becoming a Permanent Resident.