At this point in human history, slavery was as common as owning farm animals. This is no longer the case and it would be rather duplicitous to try and argue that the United States had nothing to do with ending open slavery.
Not really. By the time the declaration of independence was signed, the US was really the last bastion of slavery in the civilized world.
IMO, the argument for the 3/5 compromise was actually much simpler than that. It was simply necessary. It was the only way to unite the colonies as a single country. The mere fact that the constitution could not eradicate all evil and injustice that existed at the time isnt an argument against its validity or desirability.
What most people don't realize about the revolutionary war is that it was not, by any stretch of the imagination, a decisive victory over England. In fact, had Cornwallis received the support that he was promised by england, and/or if the US had not had the french on our side, it is very unlikely that we would have prevailed. England left because they decided it simply wasn't worth the cost (which itself was not very signifiant to them as a financial and military superpower). Had there been the perception of weakness or uncertainty (such as the founding of two different nations) england would have been back on american shores fairly quickly.
All that said, though the civil war did end the injustice of slavery, it was not about slavery. It was about secession and in many ways it was a repudiation of the core principles of the declaration of independence.
When in the Course of human events, it becomes necessary for one people to dissolve the political bands which have connected them with another, and to assume among the powers of the earth, the separate and equal station to which the Laws of Nature and of Nature's God entitle them, a decent respect to the opinions of mankind requires that they should declare the causes which impel them to the separation.
This is the very beginning of a document that at its very core was intended as an apologia for the right to secede.
FTR, the second amendment does not deserve its own bullet point. As it was written and intended, the US as a country has long since abandoned the principle behind the second amendment (which is the establishment of state level and local level militias with the aim of acting as a check against federal military power).
Great comment.