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"In all of my many years as a Christian I was taught that men are imperfect because of Adam's sin." 

this is correct.  but it does not mean that we bear his sin.  we are not born guilty, we become guilty through our own sin, the tendency for which we inherit from adam, and will bear our own sin unless we get it dealt with as you outlined. 

what you say about adam being punished by expulsion from the garden shows how christian doctrine finds an echo of sorts in islam.  if we were pure and spotless at birth, then we would be admitted to the garden straight away.  the fact that we are not indicates that allah is also punishing us  because of adams sin

we do not find infant baptism in the holy scriptures

No, Allah is not punishing us because of Adam's sin because men were created to be viscegerents of the earth. That was our purpose, so we are fulfilling that purpose.

In Islam we do not believe that God created man for companionship, as God is not in need of anything.

Allah punished Adam by expelling him from the Garden. Mankind was always meant to be earthly creatures.

If Adam had been created as a perfect man, he could not have been tempted, could he? Adam was a man, and no man is perfect.

Allah states in the Quran:

2:30 Behold, thy Lord said to the angels: "I will create a vicegerent on earth." They said: "Wilt Thou place therein one who will make mischief therein and shed blood?- whilst we do celebrate Thy praises and glorify Thy holy (name)?" He said: "I know what ye know not."

This is clearly man's purpose and reason for being created. If Allah had meant for man to remain in the Garden, then He would have stated He was creating a viscegerent of the Garden, not earth. Allah knew that Adam would choose to disobey, because Allah(SWT) knows everything that will happen. Adam had complete free will, and he and Eve alone bore the consequences of their actions. Allah did not force Adam to disobey, but He knew it would happen.

Most scholars believe that man's descent to earth happened the way it did as lessons to men. It was meant to teach man that Satan is the enemy of man, that he caused Adam and Eve to be tempted and expelled from Paradise so that by avoiding Satan and evil men can return to Paradise. They also learned that Allah punishes those who disobey Him, and that submission to Allah is the only way back to Paradise. Finally, Adam and Eve learned to ask for forgiveness from Allah, and that Allah is Oft-Forgiving for those who repent with sincerity. These lessons were necessary for mankind's viscegerency on earth.

 The Quran states:

2:36 Then did Satan make them slip from the (garden), and get them out of the state (of felicity) in which they had been. We said: "Get ye down, all (ye people), with enmity between yourselves. On earth will be your dwelling-place and your means of livelihood - for a time."

We do not know where the Garden, or Paradise was/is, but it is clear that it was not on the earth as they were sent to earth.  All ye people refers to Adam, Eve, and Satan who were in the Garden together.