Regarding the 3 fill-in-the-blank questions, why can’t the best fit for the third blank be "disingenuous?" The entire prompt talks about how their campaign isn't really true and that they may be aware of that, making "disingenuous" work quite well. GRE questions are very much "by definition" in nature and there seems to be more context for "disingenuous" than "efficacious," though both may be true. Neither the text nor the video explanation explains this.
The idea of disingenuous behavior is expressed in the passage, but that does not make disingenuous the best fit for the blank. Let's go through the blanks.
The clue, “their message…dire predictions” indicates that the advocates' message has been one of focusing on the negative facts—not the reality. Therefore, such advocates are reluctant to acknowledge good news; in this case, the fact that smoking has been been steadily decreasing.
Later the sentence says that anti-smoking advocates will thus continue to make dire predictions because they don’t want this positive trend to stop. "Remain unchanged" isn't quite positive news—you could argue that it's not bad news, but it's definitely not a positive trend. So "been steadily decreasing" is the only viable choice.
Now...let's get to the third blank:
The structure of the sentence is that the first part makes a statement and the second part—" ...perhaps aware that..." gives an explanation for the first part.
The only way the logical cause-and-effect of the sentence makes sense is to say that the advocates misrepresent the current situation and make dire predictions so that their message will be more effective, or efficacious. The advocates want to focus on negative facts because they think their anti-smoking message will be more effective that way.
In contrast, in this context, "a desire to be more disingenuous" isn't really a reasonable explanation for why advocates don't want to acknowledge that the trend is unchanged. It doesn't really make sense to say they are misrepresenting the real situation, perhaps "because they are aware that this message will be more dishonest."