It's a legitimate choice in the sense that it's a decision you can make and that it carries a political message. It's most likely not the message you intend though.
No, it doesn't. It really super duper doesn't. I mean it obviously doesn't.
There is no politician in the whole of the United States who is looking at the absymal electorate percentages and saying "I bet I can get that non-voting 70% if I form a new party!" or whatever because the system itself prevents that outcome structurally. As long as we are first-past-the-post it will literally never happen.
Your only option to shape society is to choose from the options presented to you. Abstaining is absolutely and completely not a protest, it's a validation of, and vote to continue, the status quo.
Also, do you think I'm going to sit on the sidelines if a candidate makes reforming voting, eliminating the two party system, doing away with the electoral college, reforming fund-raising and eliminating gerrymandering their entire platform (with a reasonable plan for execution)?
As someone who isn't represented by either party of a two party system, my only option to shape society is protest. Supporting a candidate that doesn't represent my views just reinforces the status quo.
Unless you mean "support" as in "I will respect whatever happens", in which case I agree. But that applies to voters as well, and pretty much everyone.
You can say "these candidates are both terrible" or "these candidates are completely indistinct" by not voting, and be saying "Candidate A will bring total disaster, even though Candidate B was the worst candidate I had ever seen until I saw Candidate A" when you vote for Candidate B.
Your false dilemma is just a militant way of not listening to people.