Why does your husband so easily sacrifice the well-being of his son so he doesn't "make waves" with his parents?

Did the in-laws ever inquire with your husband why you no longer talk to them/bring the kid(s) around, or did they just not give a fuck and haven't even noticed? If they did ask, what did your husband tell them?

To be blunt: I'm not really sure why you're confused at the situation, here. Your motus operandi was to straight-up ignore the in-laws, and your husband's was to straight-up deny and avoid the glaring favouritism problem. With you refusing to bring the situation to the forefront and your husband being more terrified of confrontation than a toddler seeing a spider, there's no possible way any of the favouritism issues could have been resolved. If the in-laws didn't notice/care they were doing it, they're not going to notice/start caring out of the blue with no instigation.

Do the in-laws know that Tod calls their son "Dad"? They obviously treat him as an outsider to the family, not a member of it, due to the child being yours and someone else's. Due to the fact that Garth is apparently incapable of having conversations that may be difficult in any way, I wouldn't be surprised if he's never brought up to them that he thinks of Tod as a son, or they wouldn't treat Tod like some kid you picked up on the side of the road a few weeks ago.

Someone has to be an adult here. Either you stand up for your son(s) to your in-laws, assuming that your husband will not have your back, or your husband magically becomes an adult overnight and tells his parents the way they treat his children is unacceptable.

For the moving thing: that's fucking ridiculous. Is your husband's plan to just leave in the middle of the night and then tell them you moved the next time they try to mail him something?