You didn't ask me, but.. One line is mistaking Mary, or any other human or object for God. This of course would be idolatry.
Also, I think the communion of saints is often a point of confusion here. Or a point of disagreement that adds to confusion over the Catholic Church's view of Mary. We believe Mary is special AND that she is with God and in communion with us (as are the other saints - known and unknown). So some, I think, infer that if we are in communion with Mary, we must believe she is God or another diety.
When does one hold her in two high esteem? I dunno. I would say that's sort of like wondering if you could love someone too much. You can love unwisely, but not too much.
So, as long as Mary is not confused for God, I think no line of esteem or love has been crossed. Mary is the Mother of Our Lord, this is the source and reason from which our love, compassion and esteem for her comes. Personally, I cannot imagine the two as separate, it was through the human Mary that God became incarnate; and the Incarnation is the irreducible focus of Christianity. Immanuel. He had a mother, just as we all do.
So, it is like asking: Could you hold your mother in too high esteem? And, when the "mother" here is the mother of God, love for her magnifies, and is magnified by, love of God. I think that is a good thing.
Thanks for posting the article.