“I see nothing in the infallible word of God that supports this.”
No doubt. On the other hand, billions of Christians have and do, the Church has found infallibly that it is so and yet you cannot find anything that supports that point of view. Odd, no?
“...only the scriptures have been declared infallible by the Church.”
Not so. There are fringe sects that argue that point of view, but the Church certainly does not.
Aren't we straying from the ecumenical idea a bit? After all, we should be discussing Mary's Magnificant from everyone's view point if we're going to have an ecumenical discussion. That means we should keep to what precisely is written to satisfy us wacky Protestants. Leaving the pages of scripture to venture into Church's teaching is voyeurism.
Now if you want to discuss the infallibility of the Church, I would suggest that is not an ecumenical discussion.