Hi C.,
Wow! You pretty much described my life. My older son, James, was born in Sept. 2005 and my younger son in May 2007. They share a room as well, so we are at the mercy of James. He will fall asleep in his bed, but almost always gets up in the middle of the night and comes into our bed (he is in a twin bed on the floor and can open doors). We thought about gating him in, but then he'd wake up the baby yelling or crying. Yeah, it's a dilemma. My husband usually can't sleep once James comes into our bed, so he ends up going to sleep in James's bed. So far, it hasn't bothered our relationship too much (i.e., sex life, right?) because James is not there when we go to bed. So, my suggestion would be to get Koltin down in his own bed so you and your husband can at least have that part of the night to yourselves.
Have you tried the "excuse me" method? That's where you lay down with or next to Koltin in his bed, then say, "excuse me for just one minute, I have to go check on the dog (brush my teeth,go to the bathroom, whatever is not interesting to Koltin)", then come back to check on him and keep doing that until you are away longer and longer. Then, he will usually fall asleep on his own eventually. It probably won't work the first couple of nights, but if you keep at it and truly come back to him in a minute, then two minutes, then five, then ten, eventually he won't think it's any big deal to stay in bed without you. I know it's really hard when the baby is there in the room sleeping while you're trying to do this, but even if there are a couple of nights where Koltin wakes up the baby with this method (maybe your husband can take the baby during this), it seems to help in the long run.
What to do about middle of the night, I'm not sure, especially when you're so tired. Sometimes the path of least resistance is the way to go at least until the baby is sleeping through the night. My guess is your baby's sleep patterns will change a lot by the time he turns 1. It seems like most people have that experience anyhow, and babies can more easily sleep through the night then. When that starts happening, you'll have more energy to get your older son to do what you want in the middle of the night.
Best of luck! Keep us posted...