Captain Talavar
1) Stewie still has a time machine. Let's not forget that before he destroyed it, he traveled into the future to get the latest Christmas presents. All present-Stewie has to do is intercept past-Stewie at the toy store, kill him, and use the return pad to travel back to the past, and then he can make sure Brian isn't killed.
2) A future episode of the season is called "Brian's a Bad Father" and there's another future episode called "Brian the Closer". So yeah. I don't think Brian's death is going to last.
If that was the case, Stewie would have rebuilt his time machine in the same episode. He wasn't able to retrieve the part that he needed to re-build it either.
The Time Machine, so far, is gone for good.