YANBU to want your husband to be a husband. Unfortunately, the man you married is inadequate.
He is a Disney Dad at best, in fact he's an UltraDisney Dad. He doesn't parent, just plays with them. There are no consequences for his children's behaviour, indeed he has rewarded them for acting up. Going to a hotel for the weekend? And that's been going on a year now? Fuck me, that is some serious avoidance! He's kicked the problem into grass so long it looks like a rainforest.
Now that you've admitted to being the main earner and that you'd "actually be better off financially" if you divorced - make it happen, please. I don't think all your MH problems are down to dealing with your child's complex health issues, I think a good part of it will be coming from being treated as if you're toxic by your SC and the unconscious knowledge that your husband would rather behave bizarerly (weekends in a hotel? C'mon, bizarre!) than be a functional adult / husband / parent. Knowing that you are completely unsupported is a drip-drip-drip taking you down. He's been leaning on you for a year, making excuses rather than parenting his children - he's drained you dry, emotionally.
"Deep down my head doesn’t think we can get through it but my heart is clinging on as we have something so special."
No, you do not have something special. It might have seemed special when all was going well, but some people cannot deal with life when it is not going well and your husband is just such a person. When his kids' visits went well, he seemed a good husband (although you must have seen his poor parenting even then). Then when his kids started acting up, he became spectacularly dysfunctional, decamping to a hotel rather than follow through consequences and ingraining in his children that kicking off works. I'll be blunt - this is all his fault, not the vague relative who was stirring - HIS. His refusal to parent, his desire to be liked by his unboundaried children - his choice.
For the love of God and for your mental health, stop this man draining you and tell him to stay at the fucking hotel and give you peace.