You can fire at any disk you want, as long as it is within range. No matter if it is pinned or pinning.
As for your second question. The rule reads like this:
• A disk overlapped by or overlapping the target disk is eligible for
scatter. The closest disk is always a disk engaged with the target disk.
If more than one disk is engaged with the target, the closest disk is the
one that is most overlapped by or overlapping the target.
This should take care of 99.9% of your issues, because of the way the disks move, there will almost always be a disk that is more overlapping/overlapped than the others. I don't see anything in the faq or rules that would help if all the disks were covering about the same amount of area.