How would it be slanderous? There are laws for slander and I don't see how it could be said that it's slanderous.
Actually, if
the landscaper lied about the murder for hire plot, that is slanderous. Whether it is actionable slander against the landscaper depends on Oregon law. Even if it is actionable, two very important questions remain: (1) Can the plaintiff afford to pay a lawyer, or will a lawyer take the case on contingency (based on his opinion of the law, as applied to the case), and (2) if the plaintiff wins, will the damages hoped for be sufficient to be worth both the plaintiff's and the lawyer's time, and does the landscaper have the money or assets to pay those damages?
Note that a good, ethical, non-brain-damaged lawyer will not take a case on contingency if he thinks the plaintiff will lose and/OR if he will thinks he will not be fully compensated for the considerable amount of time he will have to put into the case. IOW, many times people have a great case, legally speaking, but the damages they would probably be awarded won't even cover the legal fees
unless the defendant has plenty of applicable insurance, so it isn't worth it for the lawyer to take the case on contingency.
People who say they believe the story, or opine that it is true, have not committed slander. People believe all sorts of things that are untrue or which cannot be proved. Therefore, in that context, beliefs are opinions.
For example: Some people believe in ghosts. My son was astonished that all of the kids he knows at his high school hold this belief. He does not believe in ghosts and thinks it is silly to do so. All the kids, including my son, have opinions about ghosts, since neither side can prove the truth of the matter they assert. This is very similar to the situation in which we find ourselves when we discuss Kyron's disappearance.
Another example: Some people believe that the moon landing by American astronauts was a hoax perpetrated by the government. They can have that opinion all they want to, but their beliefs are contrary to mounds of evidence.