I think the Catholic church agrees that worship of Mary is idolatry. Perhaps you're assuming that simply having a statue of her in the church is idolatry?

Evangelicals never seem to account for the communion of saints. I think they assume that if they can't find it mentioned in the Scriptures, it's a sin to believe in it. Am I right about that?

"Prayer" is not interchangeable with "petitioning deity." Vestiges of this can still be found in modern English; for example, the last paragraph of a civil complaint contains a "prayer for relief." The attorney isn't worshipping the court. Similarly, prayer to Mary -- or to any other Saint -- is not worship; it's taking advantage of the communion of saints by asking someone in heaven to pray for one, because "the prayer of a righteous man availeth much."