Mormons can explain this. How do you reconcile that with:
John 3:5
Jesus answered, Verily, verily, I say unto thee, Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God.
In Mark 16:16 Jesus says, "He that believeth and is baptized shall be saved; but he that believeth not shall be damned."
Acts 2:38
Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost.
Seems pretty damn clear to me and trumps your anecdote. . .
Simple.
Paradise and the Celestial Kingdom (Heaven) are two different things for Mormons.
After death but Prior to the final judgement one goes through a mini judgement phase dividing people into prison or paradise. Paradise is for those who demonstrated faith. Prison is for those who didn't demonstrate any faith or who had zero knowledge of opportunity to hear the gospel.
So if you're an evil doer Utah fan who spilled beer on Max Hall's mom, you're for sure going to prison. But alas, you might have an opportunity there to meet Mormon missionaries (complete with ties and name tags) teaching you the gospel. This will prepare you to accept the Mormon temple ordinances such as baptism, receiving the gift of the Holy Ghost, etc.
If you refuse, then your progression is damned.
So in the book of Mark, he's referring to spiritual progression. If you have faith, continue to accept gospel ordinances (like baptism) and continue along the path then you're progressing. Those who reject Christ, reject his commandments, don't get baptized, act like classless Utah fans, etc their progression is then damned or stopped.
Hence, why Mormons practice their post death baptisms in temples thus solving the baptism dilemma. Why should someone who died without the opportunity to be baptized have their spiritual progression stopped or damned?
If those who lived before without any prior knowledge of the gospel died without any opportunity to hear about the gospel, they can be taught in spiritual prison and advance to paradise. If they choose to accept the baptism and other ordinances done in Mormon temples then they're scoring points that will look good for their report guard at the final judgement. I'm being kinda funny here, but theoretically, completing all of these ordinances aren't merely to receive a stamp on some spiritual report card. Theoretically, they're turning people into spiritual and faith filled people. Often times, people talk about their baptisms or competition of certain temple ordinances as being motivations into being better people. So IMO, if a dead person is hearing the gospel and accepting Christ into their lives in prison and accepts the baptism performed for them in a Mormon temple, they're probably turning into better people which will enable them to process to a higher kingdom of glory that they would not have otherwise progressed to.
At the final judgement everyone will be divided into 1 of 3 kingdoms or degrees of glory. Celestial, terrestrial, and telestial (Cor 15:40-42).
When Christ told the man dying alongside of him that he'd be in paradise it wasn't saying necessarily that he'd join them in heaven. But that he would be with him in paradise and not spiritual prison. Had the man been baptized before? Had the man even heard the gospel before? Who knows?
Hence, why Mormons may regard the bible as incomplete. What is known is that the sinner demonstrated faith in Christ which propelled him to paradise and not prison. Now is it enough to advance him to the highest degree of glory for the final judgement? Probably not. The additional factors need to be weighted in... Such as life circumstances, knowledge, whether or not temple ordinances are accepted, etc.
Something shady and unclear (in my studies at least) in Mormon doctrine is what the dead are doing right now. Many teachings demonstrate that there are missionary efforts underway in the spiritual world. People aren't just pulling a trout and fishing up in the clouds and enjoying themselves. Yes, you're free from physical labor (you have no physical body). But no, that doesn't mean that you aren't working in some spiritual capacity to improve upon your knowledge, attitude, and overall final report card. how that works? I'm not sure.