Does the doctrine of original sin explain our propensity to sin?

As a young fellow I once heard a good Adventist pastor contend that we don’t become sinners from sinning but that we sin because we are sinners.  Now for many reasons I loved this particular ideology but as I think about it this doctrine raises some questions which really need to be answered.

First let us define a sinner.  The operational definition of a sinner from my study is as follows.  It is an individual who upon knowing what the good is chooses the evil any how.  This is why Adam and Eve can be counted as sinners now in spite of their former sinlessness.  They knew the score and yet they chose sin anyhow.  The dictionary definition of sin is also helpful.  It is simply a person who has sinned.

So to the questions.

1. Why did Adam and Eve sin?  Did they become sinners through sinning or were they already sinners?
2. If they were already sinners then does that mean it is impossible for God to create beings that are already sinless?
3. Why did the Devil and his host of minions sin?  Did they not start from a default condition of sinlessness?
4. Paul tells the Athenians on Mars Hill  [Act 17:30 KJV] And the times of this ignorance God winked at; but now commandeth all men every where to repent:  If I read this correctly God only holds us accountable for that which we do know while holding us innocent for that which we do not know.  For this reason we safely regarding children as being regarded innocent by God until they reach the of accountability.  So at what point have children become sinners?
5. If children are sinners at birth what sin did they commit?

Now consider the following.  By their fruits ye shall know them.  What spiritual fruits do we observe in the lives of those who hold to the doctrine of original sin?  Many years ago in discussion with a Melanesian gentleman from the Solomon Islands, he observed that Adventists were the only ones who showed a change in lifestyle once they chose to become Adventists.  He noted that all other Christians stayed in  the same lifestyle even after accepting their brand of Christianity.  One atheist gentleman once challenged him asking him what possible benefit Christianity could be.  “For starters you’re not on my dinner menu” was the response harking back to his cannibalistic heritage.  Oh yes he had a dry sense of humour.

In the original sin doctrine a person will keep sinning for the rest of their life.  This obviously must include the time after the period of probation has closed when there will be no intercessor.  It would appear to me that the doctrine of original sin is going to damn many to hell because of the fact that they will sin after probation closes.

Your comments are invited.