I've heard many (eligible) complaints that in a timeline where the observers never existed September would have never distracted Walternate from finding the cure and Septmber would never have saved Peter'S life.
My personal "solution" to this:
Walter would still have kidnapped Peter, just for the selfish reason to have a son back (wich would make him even more guilt-ridden).
Only this time he would wait until Walternate has cured Peter -> the lake wouldn't be frozen anymore and Walter couldn't make the stupid decision to cross over on a frozen lake. ^^