I thought it was common knowledge that they see the rehersals certainly I remember it being talked about last year that Alesha definitely saw rehearsal footage to aid comment writing. It is the run through they see not the dress rehearsal (which will explain the previous posters comment) gives them a chance to write their own comments and views. They then watch the performance and watch different bits ( if watching footwork mainly during run through will look at top line during the show for example) and also tweak the comments depending on how affected by the actual performance, costumes, any mistakes etc.

I don't know about the marking, I assumed they decide there and then the actual mark but I guess they would have an idea from watching previous footage. Len said last week (I think) to a particular couple that the dance starts and he has a mark in mind and then found himself increasing it as the dance continued. He must have seen the dance to have an initial mark in mind.

Enough waffling, basically no conspiracy as I thought common knowledge they do see the dances during the run through