And you stopped right when it was getting good.
I like that theory though, no holes and everything can easily be explained. So if the writers did decide to take this route, I believe they could pull it off very effectively, and make it believable. The fact that us fans can write a comprehensible and believable plotline for this, makes me put a little more faith in those who are the professionals and get paid to think this up. It could be done.
But I thought about the 'flashback' idea as well, and I believe that would be something I wouldn't mind seeing either. - If they decided to bring him back for just a couple of episodes.
We've seen in season three how old cases were turned up, and Speedle was on those cases. Granted, there were holes with their plots that we picked up on, but that's for another rant. Perhaps that now we have Boa Vista, a cold case will be kicked to her, and the CSI who processed the scene will be Speedle. Now, instead of just mentioning him, maybe they could make the entire episode a flashback.
Have it flash perhaps to the case, and make it like the case B. And when if flashes back to present, the CSI team will be continuing the case. - So we'd still get Speedle, but it would be more believable that he's there, instead of 'back from the dead'. (The only problem is they'd have to change the sets back.
...Eh they can have outdoor scenes and crime scene locations, plus the Trace Lab can easily be made to look like it did in season two)
So there are the two possibilities that would work, (Back from the dead, vs Flashback episode) if the writers decided to do that for say, Sweeps, Premier, or Finale.