What level of delegation is involved by God to Mary? The image I am struggling with is that God, after the atonement, left Mary to "mind the store", in which case it almost seems uneccesary to do any prayer apart from asking for Mary's intercession. What is more comfortable for me is to view her as more of a passive channel.
The idea is not that Mary is involved in every single intercession (from us to God); we can pray as we choose: directly to God, or asking saints to intercede for us. Rather, it is that God chose her as the vessel to distribute His graces to mankind, and she always intercedes for us. To use an analogy, He is the lake; the water is His grace. Mary serves as the conduit to get the water / grace to us. We believe that this is how God designed it. He could do anything He wanted to do. We know from revelation that He likes to involve His creatures in the redemptive process. He became a Man after all. In the OT, we see Moses interceding to make "atonement" for the people. In the NT, we see Paul speaking of being "poured out as a sacrifice" for the sake of others. It's all over the Bible.
You are right to view Mary's mediation as relatively "passive." Her involvement does not in the least mean that God's involvement is LESS. This is the mistake in Protestant reasoning, so often. They see things in an "either/or" or "zero sum game" way, and create many false dichotomies, where if one thing is emphasized, something else must be lessened (whereas Catholics think in terms of "both/and"):
1. Mary helps distribute God's grace (even up to and including every instance of it).
2. Therefore, God must be doing less in the overall scheme of things than He does in the Protestant view, where Mary plays no role in grace at all.
This doesn't follow at all, not even logically. It is a fallacy. God still does it. He is the only source of grace. He's the sole cause. It is only for Him to give, because He is God; He's the one who forgives us and enables us to become more holy. He simply chooses to distribute it with Mary's participation. He chooses to involve men and women. He always does this. He gave us the Bible through men. He gave the Ten Commandments through Moses. The gospel was promulgated by the apostles. He gave His message to the Hebrews through the prophets, and announced the coming of Jesus and the New Covenant through John the Baptist. Jesus was born of Mary. He could have simply appeared as a 30-year-old man if He so chose (like the theophanies in the OT, where God appeared as a man). But God wanted to involve human beings! It shows how highly He loves and values us.
How you characterized it above, then, is not a very accurate description at all of how we view this. God is still in complete control. He gives all the grace, and it was Jesus' death on the Cross that makes salvation possible for us. Period. All Mary does is assist her Son in that process and God the Father. God does it, using Mary as a means of application. In no sense is He sitting back on His heavenly rocking chair (as the stereotype would have it; stroking His long white beard) and delegating this job to Mary as if that means He does nothing in that regard.