That is not how I remember the events unfolding.
IIRC, Walter found the cure but was too late in saving his own Peter. He then watched as Walternate was looking for the cure and distracted by September; which then meant Walternate's Peter would also die. Walter then crossed over only to give the cure to Peter. Unfortunately, he broke the vial that had the cure during his fight with Nina and so had to take Peter back to the original Universe to give him the dose. Once he did that, he did not have the heart to return Peter.
And I agree. No Observers = no Peter in our Universe. Peter would stay in his own universe; where he can have hot sex with redhead Fauxlivia and she would then give birth to Etta!
That Anubys is a GENIUS! -- Rob Helmerichs
I'm sure not a GENIUS, like Anubys -- Rob Helmerichs
And Anubys' genius just keeps shining through! -- Rob Helmerichs