The adult version of him was erased.  Imagine a line, beginning at his birth and ending at his death.  The observers just erased most of Peter's line, from the point of his original childhood death onward.  So the Peter we know was indeed erased.

This is key.  The foundations of universe-crossing were already in place before either Peter ever got sick.  It stands to reason that at some point Bell and Walter were successful in building a reliable way of crossing over, perhaps even causing the same instabilities which threatened the existence of both universes.…

That's a damned decent theory.  There have been so many plot points seemingly dropped throughout this season that I didn't think about it that way.  Could be all those little loose threads served a purpose…

I think you just hit on the major twist: WE are the Observers…  o_O

In the original timeline at least, it was implied that Red Nina had died in a car crash with Bell or something.  And I THINK we were meant to believe that Amber Nina also had a robotic arm, so the fact that this Nina didn't means she is either a shape-shifter or an alternate.  Or maybe just messy editing, perhaps,…

The issues with the Cortexiphan make a lot more sense if Nina is doing that on her own, perhaps as a means of double-crossing DRJ.

So during that final scene in the car, I was just musing on what would happen to the baby if Amberlivia suddenly merged with Ourlivia and the Amberverse started doing the same, when Olivia suddenly has to go to the bathroom.  It was such an odd character moment that it made me think she might be feeling a phantom…

The opening plane crash also called back the plane incident in the pilot (!).   As much as it saddens me to think this might be the last season, September's quite serious-looking wound in the previous episode makes me think that things are winding toward a resolution.  We just need closure on what that strange…

He did explain that they seemed to be growing double body parts.  Presumably the teeth thing wasn't specifically relevant enough to the mystery for him to mention it, although it would have been in completely in character for Walter to say "Peter, look at this fascinating anatomical anomaly!" had they been back in…

The thematic parallels you mention have been a narrative device used to brilliant effect all season long.  Almost every episode seems like a little microcosm of some aspect of the bigger picture.  Seasons 1-3 were Prelude, but Season 4 is Fugue.

Why do you need to watch a live one?  I've been living with the 8-day delay since I dropped cable last year, and though it sucks to not be able to go online and geek out right after the episode, it's worked out great otherwise.  Plus this way I don't have to worry about whether my DVR is set up properly to record the…

Whoever/whatever it was who sent Kara back had to give her something to fly, obviously.  They couldn't very well just set her spinning in her flight suit in front of Lee's Viper.  And her old Viper was destroyed, so a new one was needed. They never mention it, but I'd wager most of Kara's physical scars were also…