You may have written yourself into a corner. By defining free will to be the ability for one part of our body (the "conscious part," as you coin it) to interact with the rest of the body and the environment, you define "free will" to exist as long as one is conscious and that consciousness can affect the environment. As such, when combined with the claim that someone believes we are conscious, this directly implies that that someone believes we have free will.
The confusion you are running into is that all of the words you are using are notoriously difficult to pin definitions to, but you pinned a few definitions to them. Now you see that it's very difficult to create consistent theories using those definitions. This is the reason why there are not universally accepted definitions of those words: its hard to create consistent theories with those definitions.
This confusion is then compounded by a teleological approach to evolution. When it comes to evolution, the reason why something was selected is not specified. It is stated that the individuals who had this trait were more fit, and thus reproduced better, but it does not specify anything about why they were more fit. We do indeed seek to find after-the-fact explanations, such as claiming that Cicada's developed a 13 year dormancy cycle to outwit predators, but that is merely our model of what happened.
To understand the difference, consider a coin that is flipped 4 times. It lands heads up every time. We may see a pattern of "oh, the coin landed heads up every time." In reality, there was no "why" to this pattern, because it was truly random. But that doesn't prevent us from fitting patterns to it.
Accordingly, it may make sense not to think of it as "Consciousness exists, and we are conscious," but "We exist, and consciousness is the thing we have." The latter points out the reality that we really define consciousness based on what we have. If we had something different, we would define it different.
And so, the best reason why we can say evolution selected consciousness is because we are alive. One can go into modeling, and try to identify traits that we can use to provide an explanation for this (there do seem to be advantages to being able to be aware of your own actions), but that is merely an explanation. Nothing more.