I have not used NX wedm much but maybe we can figure your problem out. From the image it looks like you have a solid part. When I have programmed wedm for parts with taper, I use 4-axis and select faces not edge chains. You can use edges on solids, surfaces and wire frame geometry but I find that selecting faces is much more safe to use. Even on 2-d wedm paths I tend to use a 4-axis operation just so I can select faces then post with my 4-axis post. The 2-D nc output is fine, there are not any U-V moves in the file.
As for the "flipping" in your path, I have not seen this before but perhaps the chain has more edges than you need. Make sure you are chaining just a single edge and not up any vertical edges or chaining a loop, just as you would create a planar mill profile. 2-D wedm programming is actually similar to simple 2-D milling programming, nothing more. Give that a shot and see what happens to your output. Also, perhaps try using 4-axis for the Axis Type when selecting your geometry and try selecting faces rather than edges. At any rate you are going to need a working post. You mentioned editing a generic post so I assume you are going to start with a generic post in PB. Don't use the default ISO post that ships with NX if you are going to program for typical wedm's such as a Fanuc or Makino; the U-V output is not the correct format.
Have you used Post Builder before? It can be a bit daunting to a casual user and you should really have some training on how to use it, at least from another user. IMO it is not something a fellow forum poster can walk you through. Do you have support? If your support with Siemens is active you can download posts from their support site. If not then perhaps one of us could help you locate a basic post.