1. Yes, in the correct answer (option B) the phrase a greater proportion than in any previous election is a summative modifier. It sums up and describes the previous information about the television costs and what they accounted for: more than half the spending.

2. It could be argued that there is an ellipsis in option B, but my own sense of it is that there is not one. A summative modifier acts adjectivally or adverbially to describe the information before it: thus, a greater proportion than in any previous election is a complete and sufficient descriptive summation of more than half the spending. If you wanted to force a kind of greater specificity into this modifier, you could say, I suppose, that the words of the spending (before THAN) and the proportion (after THAN) represent an ellipsis because they could be inserted into the sentence.
Nevertheless, I don't really see the point of worrying about this matter: the thing you need to see is that option B has no errors of grammar, idiom, or diction, and above all that it makes sense just as it is written.

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