I can see why you have that view, it is very rational and in general true. But still, presenting your view, that you "cannot imagine that two dictionaries would have significantly different definitions" does not imply that no two dictionaries have significantly different definitions.
I made a mistake in not stating why I asked those two questions. I am interested in implementing your system of only using dictionary definitions. In order for me to do that, I feel like I only need to know two things. One: "What differentiates a dictionary definition from a non-dictionary definition?" And assuming that my answer to question one is the same as yours, "Two: What makes one dictionary definition more suitable than another dictionary definition of the same word?" The first question seems trivial, however I do not believe there is any way to answer it such that your system for choosing definitions can be implemented. Rather, the way I would answer the first question makes your system impossible to implement - due to the problem posed by the second question.
Also, you might be wondering why I'm being so wishy-washy. That is, I have my own answer to the first question, so why am I not just presenting it and going from there? If I did that, then I would be "[taking] the liberty of redefining words to [my] favour" as you say. So such a presentation would only be a pointless show of my ability or lack thereof to "win logical arguments by redefining words then forcing your opponent into logical fallacies." Hence, I wait for you to present your own answer to the first question or definition for what is a dictionary definition.