I assume that Renpy is based on Python, running a specified search function for that line should lead them to it. They'd have to recompile the files maybe? I'm only starting to learn programming less than a month ago.
Renpy is officially written Ren'py and py is because python files are .py. Rather than saying the script is able to call python, it's more like python with extensions to make it simple to handle sprites, text and other VN specific stuff. This means you can become quite creative with custom code if you really want to.
No, you don't compile python after making changes because python is an interpreted language rather than a compiled one. It means python code can be executed from source. It is technically possible to compile python and make .pyc files, but depending on what you are doing, that might not be a common approach. Not sure what Renpy defaults are in that regard. The reason why you can't just open the script files is that Renpy encrypts the files by default when building a release zip. You are supposed to play the game and explore it from within the game, not snoop around in the files to avoid exploring the game itself. This isn't Headmaster specific, but rather the standard approach for more or less all visual novel engines.
The reason why some programming languages needs to be compiled while others don't is it's easier to develop with something, which doesn't require a compiler. However a compiler will optimize the code, so it will execute faster. Renpy is using the slow execution approach because it is "fast enough", but you wouldn't want a first person shooter coded this way as that would result in abysmal framerates.
While I can say a whole lot more on this topic, I won't as it will just derail the thread. This gives a brief intro and if anybody wants to learn more, then go read programming websites and/or Renpy's own documentation, which is actually fairly good.