In your first scenario, the chance of infecting someone is 75%, that is true. However, the probability of infecting two people also exist, at 25%. The mean number of infection is 50% * 2 = 1, thus r0 = 1.
What is important about calculation of r0 is not whether you infect someone, but the number of people you infect. Why do we say when r0 < 1 we have herd immunity? It means that if one person on average infect less than 1 person in the duration then eventually the disease will stop spreading.