I wanted to get my thoughts down before I went to bed. At the end of the series, in the finale, Baltar and #6 say that everything is inevitable, meaning that when they landed on Earth and decided to start from scratch that it didn't matter that they gave up their technology. Humans would evolve to the point where they would build machines. And then machines would evolve to the point where they try to destroy humans. It happened on Caprica, on the first Earth they found, and they seem to suggest it will happen on the last Earth they find.
What I took away from The Plan was that it's also inevitable that some cylons will rebel. The Plan follows two different versions of each cylon. One of the two, even the #1 who was the main driver of the plan to destroy the humans, displays love towards humans or a human. And thus they don't go through with the plan to destroy the humans. We also saw this at the end of the series. They have a vote and half of them decide to rebel and eventually team up with the fleet.
I guess what I take the from the inevitability theme is the role of determinism in our lives. Some things are simply wired in us and we have no control over how they affect our lives. I'm a male who was born in Memphis during the 80s. Those things determine a pretty different world for me than someone who was born a woman who was born in Iran in the 50s. So in a certain sense, we are both given a plan as to how our lives will unfold. But like the cylons that don't follow through with the plan that is laid out for them, sometimes we can choose a different plan. And sometimes those people who choose that different path end up changing things for everyone else.