I was in an abusive marriage OP and I know how hard it can feel to get away but it is possible,I did and getting out of that relationship was the best thing I ever did.
With regards to your son have you thought about putting a simple lock(a catch)on your DD's bedroom door high enough so the DC can't reach it and that way that would stop your DS from being able to get into your DD's room so would deal with him peeing on her bed.
With the bathroom,would you be able to go to the toilet with him(or get the lazy arsehole to being as your having to do everything else)that way he can't stick the loo roll down the toilet or wipe pooh everywhere.
I have 2DC that are autistic(we have 5DC)and the pooh smearing is something some autistic children but I didn't jump straight to that when you posted about your DS,like pp posters have mentioned it can be caused by trauma,your DS's little world and his life have been pretty much turned upside down like everyone elses but just because he's still little it doesn't mean he's just taking it all in his stride,even more so if he's advanced for his age he could be taking in far more than you or your husband realize,along with the way your husband is treating you your son could be frightened/confused/upset but just doesn't know how to tell you all so the unwanted and for him unusual behaviours take over,it's almost like a coping mechanism.
I've found over the years that the time out step doesn't work for every child,have you tried a reward chart with him and trying to focus on all of the positives of him and his behaviour throughout the day?(bloody hard I know when your DC are blocking the bog,flooding the bathroom and pooping everywhere but the loo)it really does work and very well for some DC.
When your helping your DD with school work could you find something for your DS to do along side you?It could be anything like a puzzle,threading beads onto string,building a tower or bridge,making fluffy sheep with some card,cotton wool and kids glue,making a farm with animals with cut up pictures from magazines,feathers,felt anything you have at home.
That way he can feel like he's part of the homeschooling with his Mummy and his big Sister.
With regards to your DH I agree with the PP who said about picking up all of his crap and sticking it in a black bag for him to deal with,he doesn't want to deal with it fine,he goes without clean underwear ect,it would be his own fault and his problem for him to sort out!
Sorry the post is so long it's just I've been where you are and I fucking hate that with all that's going on there's still women having to live with they're Husbands/partners being shit!
Good Luck OP 