My deepest sympathies are with you.
I also have a 3 year old who won't nap and is having nightmares and a teething 8 month old who wakes all night too.
So sorry for your loss, finding space to grieve must be very difficult.
Being homeless is also extremely stressful, have been there.
I think the snoozing on the sofa option is one you should consider, at 3 your little boy should be able to be left in front of the TV with a snack and a drink while you get some sleep. I do this and it at least stops me feeling like a complete zombie.
Second if you can call in help, family, friends, nice neighbours you can trust or a trustworthy teenager to watch your son for a few hours in your home while you sleep. Even a few hours in a row for one day will help you recharge.
Third consider co-sleeping with baby temporarily if you can (3 year old could sleep in with his dad and you sleep with baby in his bed if he has a single bed) that way you can feed lying down whilst mostly asleep. This helped me survive when my eldest was waking every hour.
Fourth is your 6 month old teething? Calpol is your friend, if I can see my baby is going to have a bad night (sore gums, grizzling, waking lots in evening etc...) I find that a dose of Calpol around 10pm will help her through the worst pushing period for teeth at night and she sleeps better. That can get me a four hour stretch of sleep if I am lucky.
Fifth - weekends, you are owed one lie a week in by your husband. My DH takes the baby and my eldest on one morning until 10/11am and only brings baby in to me for feeding, that way I get to catch up a bit.
Hand holding and a listening ear hear when you need it so rant away x