The important thing to know is what the roll centre actually means. When you are cornering, the side force from the tyre is directed to the chassis via the suspension links (the 'geometry'). Basically, the roll centre is the point in space at which the tyre force 'points to'. If the roll centre is high, when you turn, the side force from the outer wheel will point upwards and this actually pushes the chassis up, reducing roll angle. If the roll centre is below ground, the side force will push the chassis downwards, increasing roll angle. When the roll centre is at ground level, there is no up or down force applied to teh chassis from the geometry.
Sorry, but that's not the correct way to find roll center. You're tracing the lines the wrong way and in your setup the roll center is crazy high, not low. Everything else you said is right though, just try a picture like this:
Labels: roll center