If your brother-in-law isn’t worried, then I wouldn’t be worried either. And I don’t see charges sticking either... They said they were coming to start trouble but instead they found trouble. You can’t say you’re the victim when you say you’re coming over to cause problems at somebody else’s house but you got your hit whooped instead. Again though, that’s not a designation for police to make. They just operate on the laws broken they can see and let the guys in suits figure the rest out.
And I wouldn’t worry about who the victim is listed as in paperwork either. As somebody who’s copied over about a BILLION police reports, every incident that has a complaintant has a victim. As the complaintant by default (a man clear signs of assault), that means your ex/FIL will be the named victims. You might even find that they demanded charges were pressed which, by default, makes them the victims. In mutual combat cases, there’s often multiple reports that show one person a victim in this report and then as the accused in the other paperwork. It’s not a statement of guilt or innocence or even instigator or not, it’s simply a way to track each party involved in the context of that report.
As far as the police attitude, I’m surpised but also not. Their job is to enforce and protect, not mediate family civil matters and if they made a statement of any sort then it means they get pulled into court for one party or the other and no officer ever wants that for something like a non-formal custody dispute. The rule of thumb for police is “it’s not illegal until it is,” meaning they will never say something is illegal if there is nothing they can do about it.
Your friend is right in that in most states, without a court order, you’re not denying custody but simply a Mom on vacation. Your husband can threaten all he wants but, if you’re one of the most states, there’s not much he can do. That said, there will be a point where he can claim you abandoned the marriage and start the process against you. The sooner you firm up legal separation, the better.
And no, I don’t think you lied. He did threaten to do all of those things, in writing. So he didn’t say it to your face, but he said it and you clearly feel threatened. If he had said to somebody else via text “I’m going to kill her,” would you feel threatened he’s talking like that or would you say “at least he didn’t say it to my face” and let it go? Get a lawyer to help you out, not that he denies it (and he will deny it) and makes an action on you legally that you don’t know how to respond to. Protect you. Battered wonen’s facilities can help you lawyer up. Or ask your BIL, he seems the expert.
Also, I’d leave that house ASAP and make sure the school knows there’s a protection order and Dad can’t come to get the kid at school. While you can do what you’re doing because you’re operating without official orders, so can he, which means unless there’s proof otherwise he will be allowed at the school to get your child.