In 4.21, Bell said that he has been playing this game for the last 40 years. Since the show is taking place ~2012, 40 years past would be in the 1970's. We can then surmise that Bell first thought of or began enacting his current plans somewhere in that time period.
If Bell began his master plan in the 70's, and the Observer Invasion was set for 2015, then we have a most pressing question:
-How did Bell learn of an Invasion ~45 years before it happened? Did it come to him in a dream/vision? Did he use massive amounts of LSD, whereupon he had a glimpse of a possible future? Did an Observer travel to the 70's to inform Bell for whatever reason?
Until this question is resolved, then there it's hard to take a Bell VS Observer scenario with the seriousness it merits. So if you have solutions to the above questions, then by all means lay them out.