Why there is no Original Sin

Original sin is in contrast to the doctrines of free agency and personal accountability for sin. Although the scriptures abundantly show that men will be punished for their own sins and not for their Adam's transgression, the common view is that all men are tainted with sin and denied blessings because of Adam's fall.                                            

 See Deut 24:16 The fathers shall not be put to death for the children, neither shall the children be put to death for the fathers: every man shall be put to death for his down sin.                                                                            And Ezekiel 18:20 The soul that sinneth, it shall die. The son shall not bear the iniquity of the father, neither shall the father bear the iniquity of the son: the righteousness of the righteous shall be upon him, and the wickedness of the wicked shall be upon him.

 21 But if the wicked will turn from all his sins that he hath committed, and keep all my statutes, and do that which is lawful and right, he shall surely live, he shall not die.

 22 All his transgressions that he hath committed, they shall not be mentioned unto him: in his righteousness that he hath done he shall live.                                                                                                                                                            Personal accountability for all of one's acts underlies the whole gospel plan and is the natural outgrowth of the law of free agency.   Without that personal responsibility free agency couldn't operate as neither rewards or punishment would follow the exercise of that agency.  And if there were no rewards or punishment there couldn't be any salvation or damnation, which would defeat the object of the atonement - the saving of mankind from their sins, not in their sins.  So we'll punished for our own sins and we'll be judged according to the deeds done in the flesh.                                                                                                                                                            

Rev 21:12 And I saw the dead, small and great, stand before God; and the books were opened: and another book was opened, which is the book of life: and the dead were judged out of those things which were written in the books, according to their works.                                                                                                                                                               So we're accountable for all our acts, both temporal and spiritual.                                                                                            

Regarding the flood, when everyone except Noah and his family were destroyed, the same also happened in Sodom and Gomorrah as well as with the scattering of Israel.  These are where a whole people are under condemnation, but because the children haven't sinned they will be rewarded in Heaven.  You could also say the same about war and the victims of murder.  These people aren't under condemnation.                                                                                                                                                                            

Regarding prophets: 

Basically, you say that since Jesus, the heavens have been sealed and there is no revelation.  As I have been saying for a while now, you believe in a god of confusion, one who established the gospel but then allowed it to degenerate to such an extent that it bore little resemblance to how it was when set up.  Good people came along, and without any authority set up a church - or rather a number of churches - with different doctrine.  As, in your gospel, there is no revelation, it is impossible for man to find out what is God's will.   If you say there can be no revelation in our day, there can be no communication from God, so prayer can't work.  You say that you have to get everything from the Bible, that God has spoken and will never speak again.  However, you profess the workings of the Spirit, answer to prayer, but not in modern revelation.  Isn't that modern revelation, but to individuals?  so if you believe that individuals can receive revelation today, it is inconsistent to deny the concept of modern prophets.  Now think of Cornelius.  He was visited by an angel, who confirmed the truthfulness of the gospel, but e still had to go to Peter to be baptised. He had revelations before he joined the church, but the Lord didn't tell Cornelius to set up another church.  He told him to go to the authorised apostles.                                                    

If You deny the apostasy,then you ignore the fact that the ancient apostles were killed of, the right to perform legal and binding ordinances in the name of God was lost and there was no central prophetic office left to save the saints from drifting away in various directions according to their own inclination.  There are many churches that claim to trace their existence back to the ancient apostles, but none of them is like the one described in the  New Testament.                                  

 Joseph Smith was only recording what the Lord told him, when he said that the churches then were an abomination,  repeating Jude 1:  4
For there are certain men crept in unawares, who were before of old ordained to this condemnation, ungodly men, turning the grace of our God into lasciviousness, and denying the only Lord God, and our Lord Jesus Christ.                                                                                                                                      

Also    Isaiah 29:  13 ¶Wherefore the Lord said, Forasmuch as this people draw near me with their mouth, and with their lips do honour me, but have removed their heart far from me, and their fear toward me is taught by the precept of men: