It's very possible that some of these previews/clips overlap into the same arc. If you look at Ryloth for instance, the three episodes of the arc followed three different groups of characters. All were related to the same conflict, but still different. If the Sifo-Dyas arc is 3-4 episodes and Plo Koon is prominent in at least the first episode in which he finds Sifo-Dyas' lightsaber, that does not mean that Plo Koon is necessarily the main character for the other 2-3 episodes and that the clip with Anakin and Trench does not also take place as part of the same arc (it may or may not). One thing I'm curious about though is that if the go the route of the Jedi discovering that Sifo-Dyas had been murdered BEFORE ordering the army, and then they also see a clone gun down a Jedi, how did Lucas and Filoni rationalize the Jedi returning to the front lines with clones by the time of ROTS? If they discover Sifo-Dyas was murdered and someone posed as him to order the army and now the clones are breaking down and killing Jedi, you would think that they would full stop bring further use of the clone army into question. Someone a while back posted a link to an earlier draft of the script in which it is questioned by the Jedi whether a war mongering Senator posed as a Jedi to order the army. I wonder if they will go that route, with Sidious doing something to throw the Jedi off his trail and make it look like a Senator ordered the army and drive further mistrust and disunity between the Jedi and rest of the Republic.