It makes a lot of sense for the rebel fleet to get closer to the death star. It is big and slow and has to rotate to aim its superlaser. If the rebels are far away, the DS has to move only a few degrees, but if they are closer to it, it has to move more.

And a big hole in the unfinished DS doesn't really need any explanation. The emperor arranged the trap and he was convinced that the shield and the imperial fleet would prevent the rebels from getting inside. Also, since the priority was to have the superlaser operating, they probably didn't have time to install turbolasers and such.

Thanks for reminding me about the scanners, I was just typing this from my memory. Maybe the jammer was also located in the bunker. Or the jamming signal came from one of the imperial ships that were destroyed. Or the rebels' scanners were only jammed in the beginning, as soon as the DS fired its laser the big surprise succeded, no more need to keep anything secret.