nothing stops a third party from being "viable" except lack of support.
you are free to vote for whomever you want for whatever reason you want.
people are really frustrated with everything that doesn't favor bernie as "proof of dem corruption", as though there's not people who don't care for bernie or something.
its not a valid criticism that "a party doesn't cater to my demands because they try to maintain a consistent ideology" - you are supposed to find the party that fits you, not the other way around. same for candidates.
i get that you are frustrated that what you like isn't that popular, but thats how democracy works.
you brought up the green and libertarian parties- theres a reason that parties who demand more stringent ideological purity appeal to less people have therefore have a smaller base. same reason that bernie tries to have his cake and eat it by denigrating the dems while simultaneously running as one.