I am positively not an expert on the subject but here's just a few thoughts:

I think the english language became dominant because of the total number of english speaking people being by far in the majority at the beginning when the country was still a colony and as you can see by adding up the english language nationalities, there's a lot more of them than germans. But I also think that a lot of german influence is embedded in the American culture. The first post in this thread already proves the point. Also, science and technology owe a lot to german immigrants, such as Einstein and Werner von Braun. I know there's a lot of others but I can't remember their names now.

Regarding your questions on the individual points in the list. I'll try a few. ( Remember, I just found the list by Googling)

True scandinavians both geographically and linguistically are the Swedes, Norwegians, Danes and the Islandic people as well. The Finns live in an area that is known as Fenno-Scandinavia (Scandinavian peninsula + Finland)
but speak a language that is not considered a scandinavian language. Finland was part of Sweden for about 600+ years between approx. 1154 and 1823 (Maybe my fellow Finns can correct me here if I am wrong about the years)
and most of us Finns consider ourselves to be part of Scandinavians, like most things Swedish, share the same kinds of foods etc. Also genetically it has been proved that there is a close link between the Finns, Anglo-Saxons and Scandinavians, contrary to prior beliefs. Naturally some childish rivalries also exist, as with any neighbours.
However, back to the list, I believe that the Scandinavians have been separated from the list simply because they wanted to hilite the large number of Swedes and Finns who have emigrated to the US. I guess "Scandinavia" is then those Scandinavian countries that have not been listed individually.

The Pensylvania Germans are probably the group similar to the Amish people, not 100% sure of their name, maybe it was the Quakers. But now I am guessing. Hopefully someone else can shed some light on that and the Scottish-Irish, American etc. points you had questioned.