Perhaps your question is to generic to get good answers. So let me try:
- Yes the designer is used to create your own screens
- Not sure if you can edit the default screens (you probably can but I wouldn't do it)
- Please use a more recent beta version (188.8.131.52x is the latest I believe). The designer is a pretty crappy product but it works for me, after you work around the bugs and then you can create awesome screens. But the recent beta versions contain a lot of bugfixes that make it more workable.
- Note that the designer is part of the Pro version of HS3 or can be purchased as a separate product