The point is it is the the Holy Spirit that empowers believers and carries our petitions to the heart of the Father...Christ is our intercessor before the Father. So called "venerated saints and Mother's of God", have no part in that matter. In the end, God himself does the heavy lifting, Christ himself does the interceding, and the Holy Spirit himself is the empowering of the Saints. Does the "cloud of witnesses" as Paul describes have importance? Certainly, I have no doubt we are prayed for and God listens. But to actually pray to a saint and say..."okay Saint 'whatever',grant my petition and intercede for me," is far different from Praying directly to God in Jesus name. We can ask other "saints" meaning other Christians to pray to the father with us and for us, but the Biblical pattern is that men are to direct their prayers and devotion directly to God the Father and obey his command to place our faith in the Lord Jesus Christ!
Your response is, of course, correct. We pray for each other by the power of the Holy Spirit through Christ, our one and only mediator, to the Father. But my question remains:
So do we have any reason to believe Mary...is not among the persons we can ask to pray for us?
She's either part of the body of Christ or she's not. St. Paul doesn't think death will separate us from Christ, Jesus indicates as much in Mt 22, God is the God of the living. At the Transfiguration, Moses is somehow or other interacting with Christ.
I know it appears very weird if it is not part of the faith tradition you were raised in. I know it can look as if Catholics somehow think saints who have preceeded us in death have a "separate" ability to grant petitions. They don't. And that's not what the Church teaches or has ever taught. They are just like any other member of the body, Christ is the one mediator for all of us and the head of the body.