I'm going to answer the main question of your post:
'What are my chances of getting them back?'
Right now, the chances seem low.
Reading your posts, the situation sounds like this:
The children live in an unstable home. Their mother has serious health anxiety which puts her in hospital regularly. She has even been detained on a mental health ward for 6 days.
Under the terms of the current Care Order, their father is supposed to tell SS when mum isn't around or is in hospital. He hasn't.
The children have been removed to a safe place.
Your job now is to engage with SS. You've been asking for help. Take all the help now. If you see it as a criticism or accuse SS of 'taking your children' you will get nowhere.
You have no grounds for a 'legal challenge' and talking like that will only show resistance - that doesn't help get them home.
Take your anxiety medication, properly, to give it a chance to work. Do everything else you are told/asked to do.
As much as it hurts, your children cannot be with you right now. Work on getting yourself physically well, mentally well, and able to offer the calm and safe home for them.
Good luck Flick, I hope this is the low point and you can only go up from here.