Don't get involved. If there's one thing I would suggest it's that you stay out of your parents issues, if you can. You're an adult now and it's not your responsibility to work out these things for your parents. Your father's sadness is reminding you of your sadness as a child and probably also hits close to home since you are having some unhappiness in your own marriage. STAY OUT OF THEIR SITUATION. Love your Dad, spend time with him if you choose, but try and stay off the topic of their relationship.
Sounds like there might be some issues of codependency that you are dealing with. I think you may benefit from seeing a therapist or going to a codependency support group so that you can learn to detach yourself from others a bit, so you can take care of yourself. Sometimes we have to separate ourselves a bit from others in order to do this. With him helping you with your children, is that possible? In any case, I would definitely recommend seeing a therapist. They can help you to deal with some of the issues from your childhood that have carried into adulthood.
We all have them =)
We all make choices in our lives and your Dad chose to stay with your mom. Whether it was the best or healthiest decision, it doesn't really matter at this point. He has to take care of himself and you have to take care of yourself. You might suggest to him that he also speak to a therapist to help him. But, if he decides not to seek help, there's nothing you can do about it.
Big hugs for you. I know these situations can be so hard.