"People switching from hard skeptics to fervent believers."

It's not clear to me that this should be unexpected. Skeptics can be just as ignorant of their philosophical underpinnings as believers. I was raised a Baptist. I made a quick conversion early on to atheism - I call it a brush with infantile atheism. I was an atheist for the wrong reasons. Later, I realized that I was mistaken. I declared myself an atheist, but I wasn't REALLY an atheist. I still believed the first time around. Gradually I realized that I was something of a deist. Eventually I grew to agnosticism and now I consider myself an agnostic of the atheist persuasion.

"Almost all who followed Joseph Smith had previously belonged to other well established religious denominations."
Even ardent believers can be swayed when they see something that is a supposed miracle. Besides mormons consider themselves christians - they're kinduva new sect of an ancient religion.

Overaching issue though is that in that time period (1800s) there was a lot of mystical stuff floating about. I'm not that this was a period when being tied to a single church was sacrosanct. This was a period when people believed in fairies (literally), mediums, ghosts. It was a time when people who wanted to believe felt their beliefs under attack from science. They desperately WANTED to see miracles - or at least hear of them. Legends of tablets with strange and beautiful writing on them fit that bill.

"you mean someone should be responsible for their actions?"

Most incarnations of Xianity involve a god that doesn't just punish actions. It punishes beliefs. You can do the right thing your entire life and still got to hell. In fact this is an essential part of most xian beliefs.

Leader vs follower. I'm not sure that following a cult makes a person more intelligent than the cult-leader. While many religious people no doubt believe many stupid things, it's a big mistake to assume that they are necessarily stupid peole. The individual psychology is perhaps similar - though not identical - to the psychology of the powerful CEO who is into masochism.

Two of the smartest guys I've ever known joined up with two different cults: 1 was absorbed into the JWs and the other into the church of christ of boston.