“co-mediatrix” shines the light on Jesus as the one mediator between God and man. it is not about Mary, same as Mother of God.
the problem you have with the communion of saints is sola scriptura and a misunderstanding of the Body of Christ. the saints are in Christ and are very much aware of what is happening on earth. Read Revelation, the martyrs cry out “how long until they are avenged” the angels are aware of what is happening on earth, are the saints in heaven less than the angels? again, sola scriptura robs one of the complete Faith.
“the martyrs cry out ‘how long until they are avenged”
Yes, Rev. 7:10. You might want to add 11:18, but that is a little more tenuous.
“the angels are aware of what is happening on earth, are the saints in heaven less than the angels?”
Note your own elision here. The angels surely have knowledge of what transpires on earth, but where does it say that the saints do?
Look, the bottom line is this. When Christ Himself invites us to pray the Father directly, John 16:23-27, and assures us that He will hear and grant whatever we ask of Him, why would we want to invent ideas that detract from that? Do we not believe Him? Do we not trust that the Father hears? Gives? Jesus explained a great deal to His disciples about prayer from the beginning of His earthly ministry (Matthew, Sermon on the Mount) to the end (John, on the night in which He was betrayed). Why when He taught and encouraged us to pray the Father, and Himself (e.g. Matthew 9:38), and said nothing of praying to the departed saints, would a disciple of His decide to do differently?