The export to RIS is the problem – the correct fonts info isn’t there, if you open the file and look with notepad or word, so endnote can’t magic it back? JStor needs to either download the actual unicode to the file, or change their output to include your needed fields in the bibtex file. The German title had an abstract.
Maybe someone has an alternative. Can you download it from somewhere else? Can you download them both and then combine the duplicates? (manually, as endnote can’t combine duplicates).
But I note now that the two Greek imported versions (RIS vs Bibtex), also reverse the first,lastname order of the author. Not sure which one is correct?