Probably the simplest solution would be to hire an attorney and sue your father-in-law for defamation. You certainly have good reason to pursue legal action against him.
However, my feeling is that your father-in-law's problem may be a medical one - and I recommend strongly that he be evaluated by medical experts. Depression, prescription drug use and abuse, a sudden change in hygiene, combativeness, theft, false accusations against those formerly close to him. . . . all of this suggests a mental/medical disturbance of some sort. The fact that he stole one of your husband's firearms is especially worrisome.
I am not, however, in the medical field, and hopefully doctors or nurses will post soon with their comments and suggestions.
It sounds as if you DO have some legitimate legal avenues you could explore, but I am not sure a legal route is the proper one to take here.
Good luck.