Un-rules question. Kindslaver says that "neither player" may advise the outside-assistance person. Since "neither" only means two things, then does that mean, in multiplayer, the players who neither control Kindslaver's ability nor are the person being controlled, are able to advise?

No one in the game should advise the person. The template just isn’t as multiplayer friendly as we are these days.

With Kindslaver, you can't advise, but I'm assuming you also have to stop them from taking illegal actions (casting a spell with insufficient mana or attacking with a creature with Defender, for instance)?

You can stop them from doing illegal things.

With Kindslaver, can you advise them before they start that turn?

Nope. No advising.

I am playing a game with my friend and Kindslaver him. My fiancee agrees to play his turn but also has no idea how to actually play. What should we do?

She does the best she can. She can always just end the turn.

For cards like Better Than One or Kindslaver, can the person you choose from outside the game refuse? Do you choose someone else if they do?

The person from outside has to agree to help. If the first person says no, you can choose another.