my younger brother is mentally and physically disabled. He has some really wonderful people who are lds who help him out. I don't know what we would do without them. My brother has friends from his childhood STILL and he is in his 50s who check in on him, have him for dinner, fix his truck, help him with his horses. His horses live in his best friend's corral free and have for years.
These people would be good people inside the church or out. Most of the ward treat my brother well, too. Whatever else I think about the lds church, if all the lds people were like my brother's friends, it would be a good church.
I knew a mentally retarded woman in a previous branch my children and I attended that the church refused to let be baptized, though she wanted to be earnestly.
They said she lacked the mental capacity for accountability, so she wasn't allowed to be baptized. She came to church regularly. The missionaries taught her the lessons, gave her rides each week, etc.
But still refused to let her get baptized. It was really quite cruel of the cult to do that to her like they did. I felt really sorry for her.
My heart anguishes at your question as presented. The church offers nothing for those people in question. The "problem" is on the shoulders of the most kind-hearted and empathetic people within the LDS church.
I am overwhelmed with emotion and indebted to their kindness. But they are not unique to the Morg. There is a certain percentage of society that has compassion and understanding, independent of their religious affiliation.
Kudos to those people. You know who you are. They are found in the comments to this thread, amongst many others on this site!