As we struggle through Ethics, we have to remember that Ethics is a record of Aristotle's lectures to his students at the Academy in Athens…rather than a work composed only to be read individually.
So it's natural that Aristotle would slip quite easily between the first-person plural and third person in his demonstrations: he was talking to a big group of eager-beaver philosophy students.
Take a look at how this works:
Now, to examine thoroughly all these opinions is perhaps rather pointless; those opinions that are especially prevalent or are held to have a certain reason to them will suffice. But let it not escape our notice that there is a difference between the arguments that proceed from the principles and that that proceed to the principles. (1095a31-33)
Here's the deal: the use of first-person approximates the speech of conversation or addressing someone (in this case, it's a professor addressing his students). The third-person's there to allow Aristotle to expound or explicate on his thoughts—it gives the sense of a remove from the subject matter. The third-person is a little more aloof, and a little more detached.