Crime Scene
In fact, a person can be convicted of murder even WITHOUT an extant corpse. The mere fact that a person has dosappeared, plus evidence of, e.g. blood stains, plus motive on the part of the accused are quite sufficient.
Here, however, we don't KNOW that a person has dissapeared - Joe Oort is rumoured to be invisible, but may just be faking it for tax purposes.
Your question remains valid, but your approach seems poor. I would suggest that you propose some credible alternative - that is what happened with all the other theories that you mentioned - they were displaced by a BETTER theory.
Please suggest one...