Starting with the second blank, the author is saying that elections are not a test of voter's ability to follow complicated instructions. Instead, they are a way by which voters express their preference. The second blank has to be parallel with "complicated", so confusing is the best answer.

Usability means the ease of use of a device, so if some folks arguing that voters only have themselves to blame if they fail to negative design flaws, then they must "dismissed" the importance of usability. They discard the importance of so called easy to understand design and voters should be blamed if they fail to navigate design flaws.

No legitimate public purpose is served by designs that (iii)___ voters' choices.

The last sentence is saying that there is no incentive served by designs that affects voter's choices. The author has only talked about how design flaws can impede voters ability to vote, so the third blank should be similar. Hence, distort is the best choice. (there is a typo in the choices)