c) is the simple past with the correct corresponding adverb 'then' meaning 'at that time'.
b) is either a mistake of tense or adverb, or it might be an example of a sort of historic or dramatic present. In this, the present tense verb form is used to relate events in the past, to make them more vivid. But in this instance it is not the verb, but the adverb 'now' that is in the present: 'now' means at this present time, as in a).
That's why I think it might be a sort of of dramatic present.
If one thinks of 'now' as meaning 'by this time' it would read "By this time she was President Elect". 'By this time' would refer to a time that has just been mentioned.
"She then fell passionately in love with Elias "Blogsie" Bilblow, a semi-literate lay-about, and wanted to marry him. But by this time she was President- Elect and even she realised that such an unsuitable marriage was out of the question"