I am the (brand spanking new) hinge in a V and I've thought about this question a lot recently.
Because both relationships are very new, I was initially treating them as separate entities that don't affect one another, but I realized that I was wrong.
Both affect me.
If I grow and change because of my interaction with one, then two will see/feel/experience that growth and change. But I think you can take the whole concept out of a poly framework and realize that we all continually grow and change as a result of the friendships, relationships and interactions that we experience. None of us exist in a vacuum.
So, yes, your second example scenario wherein Harriet recognizes that she really dislikes being interrupted can certainly be true. But so can the converse as TruckerPete pointed out. That comes down to how we choose to process what we learn as a result of interaction with others, whether they are close intimates or merely acquaintances.