If their brain is uncapable of thinking in a right way, they wouldn’t be the only ones that have a different understanding. Baptized babies or babies that die before birth while their parents wanted to baptize them, some people with other kinds of brain damage, etc., also wouldn’t be able to develop a spiritual life.
When someone dies, their understanding is different from what we understand while alive, but our will doesn’t change anymore, but if baptized they are cleansed of original sin, however, the problem then would be the will of the person, so I think there would be some differences in the way they act in heaven, as there is difference from the ones with roles (like apostles, prophets), the ones that didn’t follow the smallest of the commandments and taught to do the same (as expressed in the Bible) or the people who go to Heaven through purgatory.
But because their understanding is different, then the ways to develop the soul you ask about would need to be different too. I guess this is one of the least developed research types since it seems pretty hard if not impossible for most. And to know if they are useful (not talking about baptism) for these persons, we would need Divine Revelation, probably like what the saints received.
