Hi,
This sounds utterly exhausting and unsustainable. I dont know how you're functioning.
I don't really have anything different say than any of the other posters but for what it's worth...
I would absolutely get a sleep consultant, they will have faced this issue before, have methods to suggest and will support you which I think you'll need. As you say you've tried everything and none of the suggestions on this post sound like like answer to you so it's time to bring in the big guns. It is expensive but I would prioritise this above most things, better than a holiday, forget birthday presents for the next year, eat vegetarian for a couple of months, whatever it takes. Your sleep is worth everything. When you're exhausted it is difficult to see the wood for the trees and make a decision on what strategy you're going to go for so having an expert will hopefully guide you down the right path.
Is there anything you can think of that may have triggered the change 4 months ago? Starting nursery? Seems like massive separation anxiety. Is he clingy to you during the day? I would try and address this by trying to get your ds attached to a comforter of some sort. When my son started nursery he was so anxious and really needed his comforter more than before. You may need to give it to him consistently for a month for him to take to it. Every time you have a cuddle, at nappy changes, in the push chair, when he gets in bed with you, when you say good bye at nursery... just constantly shove it in his hands and hopefully he will start getting attached.
I feel like if you're going to let him CIO it would be beneficial for him to have something else to cling to. He obviously needs the security.
My 16mo boy is happy to self settle and for the most part sleeps through the night. Every so often we have 2 or 3 nights where he will wake up and not want me to put him back in the cot, I guess it is developmental regressions. When this happens I sit in the dark on the floor next to the cot, making shhhhh noises while patting the mattress rhythmically. Sometimes this will take over an hr for him to get back to sleep and I know he would go off to sleep quicker if he was lying on me but I'm terrified of starting something I can't carry on with.
I also wouldn't poo poo looking at the routine, even if it is for your own benefit. I love having a routine, before I had a baby I thought it was ridiculous that parents worked their day around the baby but it gives me structure to my day and predictability in how my son will react/behave. Sounds like you will need to largely adapt to the nursery routine. How many naps is he on at nursery? At 12mo he will likely still need 2 so if they are only giving him 1 this may need to be addressed.
Good luck xxxx