I hope no one jumps down my throat for suggesting this but have you thought about being a bit tougher on your youngest in regards to sleep?
My youngest was exactly like yours and I was not coping. I booked into a sleep class for him and it was held at the hospital and I literally could not get a park!!! I drove home in tears and my mum said forget it you just need to let him cry stop giving in. I was at my wits end so I took her advice. I watched the clock every time I put him in his cot determined not to go in for a least x amount of minutes depending on how long it had been going on for and I only picked him up if he was very distressed but would usually just go back in pat him put his dummy in and say its night night time and leave him again. It took three days and he was in a routine it was like a miracle!!! The key is you have to stick to it but and it breaks your heart hearing them cry out but in the end I felt like it was just as good for him as it was for me because he needed his quality sleep. I know this is not for everyone but.