Doesn't the first point have a probability of 1? It's the subsequent ones that become less and less likely.
To remove that constraint, so it's just any band, I think it should be more like: given a cathedral is in a particular place, how likely is it that six other cathedrals fall in a 50km wide band aross Europe.
I think if you set the probability of the first one at 1, then the rest works perfectly at any angle of band. I could be wrong, but intuitively that seems correct.
Also, the probability of the first 2 points will always be 100% because they define the line.
Also, it's not 7 random points, it's 7 of thousands of random points.
Ah - I missed this one.