At this point, you need to "shoot first and ask questions later."
By this, I mean get an attorney as soon as you can and discuss what can be done to:
(1) Document this latest incident (e.g., through a police report or some other means).
(2) Establish a safe place for you and your son to live (I think you should push for your wife to move out and leave you and your son in place).
(3) Confirm primary or sole custody of your son to you at least for the time being.
If you wait to do these things, the situation will almost certainly get worse, and you should expect your wife to generate a whole host of allegations against you.
If she is willing to ignore marital and familial boundaries by entering into an affair with one of your relatives, and she is further capable of verbally and now physically attacking you over off-the-wall, insignificant things, you should expect her to have no problem lying in court papers.
As for your wife's parents, I strongly recommend that you do not get cozy with them. You are playing with fire. In-laws almost always inevitably side with their child, even if they initially seem to be siding with the "wronged" spouse.
You have tried for many months now to be supportive and to do all the nice DB things, but it has only angered and emboldened your wife. You can't say you didn't try, because you tried very hard and very admirably. Now it is time to bring your wife to heel. This may be the only thing that saves your marriage. If not, at least it will save your son much pain and anguish. He needs you to be strong.
I strongly recommend you Google and read about parental alienation. There is a huge danger of that here, with your wife poisoning your son against you. Don't let it happen.
I wish you the best as always, and will continue to follow your sitch, even as it continues to kill me vicariously.
