[Spence]

R1 R2.
Ball hit to the outfield.
R1 goes to third and rounds third.
R1 touches 2nd starts to head towards 3B but is obstructed by the SS. R2 keeps on going. However, he sees that his teammate has stopped at third base. Runner then tries to retreat to 2nd base but it as tagged diving back in.

I know obstruction is used to protect the runner to the next base they would have gotten to. However, if it ends up that base is not available is the runner still protected and therefore put back on second base?