You have enough grounds to make a prima facie case of defamation. It will need some massaging of complaint but it is doable. Don't expect the case to lead anywhere (conviction, winning compensation) though. If you are only interested in harassing the other side by making them defend the case, run around, spend money etc. then you are on right track. All court cases in matrimonial matters are just about harassment and posturing anyway.
Filing case has some benefits for you:
(1) It will give your angry feelings an outlet. Rather than seethe in anger and have dreams of violence against your in laws, you can focus energy, time and money on drafting complaint, following up with lawyer, attending dates etc.
(2) It will anger and frustrate your opponent. Matrimonial disputes in India only come to an end via mechanism of outrage fatigue. Both sides need to get tired of the fight before the fight ends. Your case will help move that cause forward.
(3) You have only seen defendent side of Indian legal system. It is easy to believe that it sucks to defend, grass must be greener on other side and complainants just dance around in glee everyday. By filing case you will see the reality that complainant ends up with more pain and frustration than defendent, the legal system is ruthless and indiscriminate when it comes to dishing out frustration.