Hi Radhika,
My parents didnt have to go for the interview as they were above 60. In your case your FIL is 60 so he is eligible for a drop box, your MIL can file along with him. Going alone for MIL should not be the option, as it doesnt change the previous situation.
What you can do :
1) if they want to come now while your FIL is on job, chance of getting the visa is higher as he can say he has job back home and has to return. Catch in this is he should only make an application for 2 months, assuring them that he would be returning back. Ask your mom in law to apply with him. Cosulate may only give visa for 2 month or consulate may approve for longer time. Getting visa doesnt necessarily mean that your FIL has to come. The only hitch in this is that he has to get approval from AirForce.. not sure how complicated that is?

2) When they plan to come after Dec.
He can go ahead and apply same in drop box. In your documents and invitation letter you should request them to come to USA for 4 - 6 months and not 6 months. Chances are that they get 10 year multiple.

3) If MIL applies alone, she should on Rs20 stamp paper make an affadavit stating her the reason for her visit, properties, children back home, husband... all these ties she needs to show. She will have to go for the interview again. This might be risky.

In every case they should make an affadavit stating their ties in India should show a son or daughter back home, stating they are studying or are unmarried and they would have to come back to arrange for their marriage.

After all this even if they reject the drop box, then they would ask them to come for an interview. SO I believe the best option is if they apply together using drop box before dec. REST IS LUCK.
Take care and all the best.
Uma. Radhika replied. Thanks uma for the details...will be discussing this option with my husband lets see which works best..thanks a lot for the info..and the detailed mail.
Will buzz u if i have more ques..
Take care
Radhika