I agree that in all cases 5050 is not in the best interests of the children but that is not what he's asking for. If he wants an arrangment where he has the children X amount of nights and you feel that's a reasonable set up, then that's what he gets, but make it very clear that that includes school runs, after school care, getting them to clubs etc. When they're with him, he is the parent with responsiblility for them.
You are not a PA or a taxi service on the end of the phone, and as hard as it is, and I hear you one hundred percent, trying to throw yourself between the kids and their dad to protect them from his moods isn't sustainable, and you will drive yourself into the ground trying.
You need to accept that you are getting divorced from this man, and all that it entails. You can't protect your children from his shortcomings, this is their story, this is their dad. You no longer exist to make his life easier, and if he wants a relationship with his children he is going to have to make compromises with his time, readjust his priorites and work for it, like you do.
Water finds it's own level, this is a long game, your kids are still young. It sounds to me as he won't actually be as keen to have them as he's currently professing as time goes on. Sit tight.