This whole thread reminds me a lot of my parents marriage. They are 20 years older, but it's all so similar, in so many points. They're still married, btw.
The difference is that I was the one bullied at school. Sorry for asking, but why did you decide homeschooling would be the best solution for the bullying? I never had this option, just stayed in the same school and eventually learnt to deal with things.
You house sounds utterly claustrophobic, with the 3 of you stuck inside all day. (a bit like my parents house today)
It seems to me that maybe the three of you could benefit from dealing with your issues individually. Your DH seems depressed. You seem at the end of your tether, idealizing life without DH. I'm not saying you shouldn't split, I'm just saying separation won't solve all your problems.
My parents are in a situation in which they just don't have patience or kindness to each other anymore. Mum gets more anxious and frustrated, Dad gets more cornered and unhappy every day.
However, even if they separated, the problems would not disappear. The damage is done, they have to try to fix it - together on separated, doesn't matter. (In your case, the effects of the last 15 years won't disappear when you walk out the door). My mum is controlling and wants to decide the best way for my Dad to live his life. Dad is passive aggressive and instead of explaining his feelings to her, just dodges her and ignores her (justified) concerns. It's a sad thing to see.
I grew up in this environment (well, it got worse after my dad retired), and now I have trouble to establish a different dynamic with my own DH. Your son is probably picking up on all this. I'm intrigued with the bullying thing: maybe you should try to address that, instead of taking over his education, keeping him indoors and away from the problems? Perhaps it's a good temporary solution, but in the long term how will it help him deal with the arseholes we will certainly come across in life? What is he learning from the role models he has at home right now?
PS: You're not responsible for your DP happiness. It's hard to detach when someone refuses to get treatment for depression, but I can talk about my dad's situation: I keep telling myself there's only so much I can do, without ruining my own life.
PS: If I come across as rude, or horrible, or whatever, I'm sorry, it wasn't my intention.