Her siblings also have American citizenship as their father was physically present in the United States for more than five years.
https://travel.state.gov/content/tr...isition-US-Citizenship-Child-Born-Abroad.html
About the titles nothing has become official.
Carl, Oscar, Sigvard, Carl Johan and Lennart either renounced or were denied the use of their former ducal titles after they were removed from the line of succession and ceased to be HRHs. If precedence has any weight on this matter, the same rule should apply to Madeleine's children then.
It is also worth mentioning that titles like Prince Bernadotte or Count of Wisborg, which former princes of Sweden assumed after being excluded from the line of succession, were not Swedish titles, but rather titles of nobility granted in other jursidictions like Belgium or Luxembourg.