Agree with comments: there is no thing that is a “fair fight”. All that matters to me is MY survival-if that results in the attacker’s demise, so be it.
Don’t put all your eggs in the eye gouging basket. That’s relatively easy to defend against.
Similarly, kicks the the crotch are not as effective as one may hope. They’re bad, but not instantly disabling in all cases. And I think in the OP situation, there might be a leverage problem.
I might avoid biting for fear of infection, but I’d definitely gouge eyes, hit the groin, stab, shoot, bludgeon, run over with a car, etc.
“Er…I mean, twitching is a form of movement, right?”
As to the OP: There’s no such thing as “dirty” in a real fight.
“Now, sir, I advise you to be on your best behaviour or I shall have to give your eyeballs a stern poking!”
Fucking vampires. 
Seriously, my understanding is that the carotid/jugular are buried well in the neck; you’re not likely to come away with much other than a mouthful of skin. You might make a psychological impression, though.
I’m on the “dirty-fightin’” train with the rest of y’all. A fight’s serious business, and if you’re not doing it with another gentleman in the dojo or ring, your concern should be to end it decisively as soon as possible. I’ll also note that I’m personally of the Malcolm Reynolds/Tuco … Ramirez school of fighting: I’m not interested in macho bullshit posing. You start talking shit in such a way that it’s fairly clear you mean it and I don’t have a safe retreat, I’m not going to talk back, I’m going to get in the first shot. (Or, as Tuco put it, “When you need to shoot, shoot–don’t talk!”) I’ll also note that a) this probably isn’t the legally defensible route, b) I’ve never been in a fight in my life and c) my first choice would still be to retreat to safety.
Better tried by 12 than carried by 6.
Also in that scenario it was multiple attackers and/or they were attacking both me and someone like my grandson. In a one-on-one confrontation I agree that stopping would be best once the person is disabled. Less legal trouble that way.
Of course my working assumption is that I’d probably have my ass handed to me as I’m 59 YO and have a mild heart condition. On the flip side, I do exercise daily, but I’m still no match for many 20- to 30 year olds.
That’s the idea. The bite to the neck isn’t about killing your opinion; humans in their current evolutionary state are not well equipped for that. It’s about scaring him the fuck away.
Popular idea; I’m not entirely sure it would be all that effective in this particular form. (Especially not against a person who isn’t necessarily rational to start with, as your OP suggests.) Hard to judge. I’ll give neck-biting this: As psychological warfare goes, it’s less likely to result in you hurting yourself than an old friend’s favorite idea, a straight razor. On the other hand, a straight razor may have a more powerful psychological effect on mere brandishing. On yet a third hand, you probably stand a higher risk of being shot while merely brandishing a straight razor than baring your teeth and making chomp-chomp noises.
I’d try not to fight dirty at first. In fact, if I can subdue the guy physically without seriously hurting him, I will.
I dunno. Is he armed with anything? Like a Mountain Dew? 
I’d be like George Costanza: Pull hair, poke eyes, groin stuff. Whatever it takes
I’d do anything I could to fight back, up to and including the eyeballs, the kicking and the biting. If I did something while fighting back that caused them to die, tough, they started it.
Minimum and proportinate force necessary to win the fight and survive.
Which maximizes the damage you take.
I’m a big ol’ coward, and would prefer not to fight. I’d do pretty much anything to avoid a physical fight.
If I had to fight, though, anything and everything goes. Eyeballs, crotch, whatever’s necessary. Do my best to subdue the guy without worrying what kind of injury I’m causing him, then run away if possible and call the police.
I’d tell him “That’s it, you’re done. You had a chance to go home tonight but you blew it.” When I was done there would be a crowd of people holding up their phones taking video of the whole thing.
Nah, just kidding. My real answer? Krav Maga.
Works every time.
I’ve never been in a fight in my life, except on a football field. I’ve said for many years that I pity the person who ever does push me into a fight because they’ll likely get decades of pent-up anger and frustration in one go. One of us is going to the hospital.
whatever it takes. Eyes would probably be plan B if I lasted that long. I’ve never gone eyes first but have certainly done neck and groin first.