Bob, you're making me blush.... And you give me far to much credit. The way I see it, we all find little clues and pointers that lead to the end result (if we are lucky).
And that is also the great thing of a board like this, we all cooperate.
As far as the details on Mannes Egbertus 1828 go, I can't find the death of Theodora Mittendorff, at least not in Avereest. And since Alexander is born there and it is mentioned as Mannes Egbertus' residence in 1865 when he left, it only makes sense she died there between 1862 and 1865.
Unless she came to the US as well and died there... But then, Mannes had only 2 traveling companions, so if she came, one of the children must have been left behind to join them later.
So I will go on searching for the death of Theodora, she may have died somewhere else.