You don't have to depend on those of us who participated in that thread to have gotten that right. The Fiore paper http://arxiv.org/abs/hep-th/9508018
This is the case that we were all frustrated with. Fiore offers technical arguments to the effect that it's impossible, but all of us felt that it should be possible to rule it out on more straightforward grounds. If those fermions in your diagram are massive, then this post http://www.physicsforums.com/showthr...=512811&page=4
by PAllen offers an argument against the decay that I personally find persuasive.
You may also be interested in this: http://physics.stackexchange.com/que...less-particles
Re your question about [itex]2\gamma\rightarrow4\gamma[/itex], I don't see any reason why it couldn't happen. There isn't any requirement for the decay products to be collinear as there is in [itex]\gamma\rightarrow3\gamma[/itex], so the phase-space volume doesn't vanish. But it has six vertices, so I would expect it to be very low in cross-section compared to [itex]2\gamma\rightarrow2\gamma[/itex].