But I believe we can make a good case that Paul knew of a number of eyewitnesses to the Resurrection, and had talked to 2 personally. Natural explanations really aren't that attractive. I think we can count on the Romans to have made sure Jesus was dead. A hallucination in the normal sense doesn't seem too attractive. Cognitive dissonance can explain lots of strange things.
People can imagine they have seen what they want to see, as any magician can testify.
However that requires you to think that the disciples were in a frame of mind where they expected to see Jesus raised, which I personally don't think likely in this case. At any rate, I don't think you require a proof of the Resurrection. You're more concerned that it's somehow been disproven. I don't think that's the case. However I would not call them Christians in the sense I've come to know what a Christian is.