I've only been building flex for a couple of years so I haven't seen/heard all of the possible phrases yet, but there are a few variations to the language used in the rigid world.

Regarding question one, yes they are asking for selective plating of only the pads and holes.  It's frequently referred to as pad plating.  Why you need this is partially covered in your second question.  The electroplated copper has a different grain structure and is less flexible than the rolled-annealed base copper and the differences in flexibility does increase the likelyhood of cracks during dynamic flexing.

I haven't heard the term bend seam lines before but I suspect that it's attempting to designate the areas where bending and flexing is supposed to be taking place.  Where you have the flexing, you want to have all of your circuit running perpendicular to the bend with no angles or sharp corners.  This again relates to grain direction and stress points.  In the bend areas, you want to have only the base copper, nothing plated on to it that might cause a crack.

Hope this helps.

Bill Anderson
Kinetic Circuits