Lucas' own viewpoints regarding what the Force is and is not has changed several times since the first film, mostly in accordance with his own beliefs and philosophical viewpoints. In the 70s, New Age mysticism and Eastern philosophies, like Buddhism and the Tao, were very popular, and so Lucas' presentations of the Force and the Jedi were greatly colored by that. As time went on, religion and philosophy began to take a back seat to science and reason, and this, in turn, changed the way the Force and the Jedi were presented. This is how we went from the Force being "...what gives a Jedi his power. It's an energy field created by all living things. It surrounds us, penetrates us, and binds the galaxy together" to a genetic condition (midi-chlorians).
Depends on which era of the SW universe you want to focus on as to which version of the Force you're dealing with. When it comes to questions of "canon"... well, there's an entire schema of what is and is not "canon", but almost all of it with a SW label falls somewhere in the canon.
Midichlorians aren't the force. It doesn't explain the force at all. The more midichlorians you have the easier it is to feel the force. It isn't the force. Midichlorians cannot exist without the force. The force is beyond them.