If Michael Fielder's story was true, then we have another reason in support of Sheila as the killer, don't we, and if Jeremy is lying about selling photographs, that is no proof that he is a murderer. If, on the other hand, Michael Fielder's story is false, it does not follow that Jeremy is innocent, it merely serves to establish what we already know, that journalists are scumbags; and, it may be that Fielder lied because Brett and Jeremy themselves were a couple of scumbags.
Certainly, what we can say is that Jeremy's moral orientation took a turn for the worse under the influence of young Brett.