Will I see it when I try to import it?
Or when I try to compile the sketch that includes the library?
How will I know if any given error is in my library or in my sketch?
It tells you the filename and line number for the error.
Does it make a difference if I edit the library in place in the library folder itself?
Make a difference compared to what? If you edit the library within the library folder, it alters the library for any sketch that uses it. You can make a copy of it and put it in a specific sketch's folder if you only want to make it differ in a single sketch.
Do I need to restart the arduino ide each time I make a change to the library code?
Not if its just within the file. If you start copying/moving/renaming files and directories, you'll need to restart.