Careful BB! Hes the Prince of Rape and Hail Sithis. Also I thought might like this video
I was obsessed with Mara when I first started playing Oblivion. You may recall that the Ritual Sign in that game had the powers The Blessed Word and Mara's Gift, while the Ritual Stone granted Mara's Milk and Mara's Mercy. Consequently I had many Imperial paladins and priests who took her as their patron.
My "quest" to understand the connection between Mara and the Ritual led me into ES lore.
To orthodox branches of Christianity, (Catholics, Orthodox, Anglican etc) Mary represents sorrow and love, because of the agony of watching one's child suffer.
She called herself the Handmaid of the Lord, (Handmaiden of Kyne anyone?).
Catholics and Eastern Orthodox believe that they can ask Mary and all the other saints to intercede for them, because they are with God. She is known as the Mother of God, and many people refer to her as their mother as well (Mara, mother goddess).
And the final aspect is the statues of Mara, especially in Skyrim. They simply look like the weeping Virgin Mary.
As far as guiding souls to the afterlife, I don't believe that Mary does that in a direct sense. However, rituals associated with Mary are praying the rosary, consecration to the blessed Virgin Mary, and Acts of Reparation.
These rituals are intended to make people more devout, to reach a deeper relationship between God, and to save people from sin. Mary is said to pray for the forgiveness of sinners, and thus can be seen as one who helps souls attain eternal life (the afterlife, Heaven).
As for Mara and the Ritual, remember that this may not come directly from being based on the Virgin Mary. Although, Mara's milk sounds similar to the prayer where God, or the Virgin is asked to take the person to their bosom, that is, to comfort and protect them. (hence extra health is given perhaps?)
Honestly, I don't know enough about it to form a serious reply without inadvertently trouncing on someone's personal faith. However, I see no real connection. I listed the rosary and the Hail Mary because that's the only ritual that I knew of pertaining to Mary. I do know that the prayers ask Mary to pray on their behalf.
I'll have to keep digging. You said that you tried to understand the connection between Mara and the ritual. Did you come to any conclusions?