>>Where Astor was last seen would confirm where he was during his final moments,<<
Uhhhhhh...actually, no it wouldn't. It might give us a good idea, but in the end it only speaks to where he was last seen by somebody who survived.
>>and it doesn't make sense that he would go back into the ship when it was sinking so rapidly.<<
Doesn't make sense for people to rush into a burning building either, but it happens. Still, I tend to doubt that Colonel Astor did this, else he would likely have his grave on the bottom of the North Atlantic.