I haven't messed with this stuff myself yet, so I'm sorry about not being able to answer your question about how to revert back to the original core quest coding.
as to your second question, oh yes, people can play with modified quest core code, as the core code is just a library, you can design your own entire quest engine core code if you want, and all you got to do is to add it into your code (and also include it in the DL for others to play, as they need the library file for the quest game to access it via the code).
You can add any library (your own library or a modified quest core library) in the GUI~Editor or via Code. I'm not exactly sure how to do it in the GUI~Editor, but I'd imagine there's some option to "add a library" or whatever, and as for with the code:
<asl version="540">
<include ref="English.aslx" />
<include ref="Core.aslx" />
<game name="Testing Game Stuff">
<gameid>79aa8756-8d11-431b-a716-812f8403ede7</gameid>
<version>1.0</version>
<firstpublished>2013</firstpublished>
</game>
<object name="room">
<inherit name="editor_room" />
<object name="player">
<inherit name="editor_object" />
<inherit name="editor_player" />
</object>
</object>
</asl>
if you notice the:
<include ref="English.aslx" />
<include ref="Core.aslx" />
these are libraries, one is the core game engine library~coding and the other is for the english language library~coding. so to add, just write in your own library's (-ies') names in the same syntax~format~way within your game code
and for both the GUI~Editor and Code way, you need the actual library file itself to be in the same folder as the quest .exe file, for it to be able to use~include ("reference") it.
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there's documentation on how to make and use libraries, though if you can't find it, let me know, and I'll find it and provide the links for you to it.
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There's also a little tidbit about libraries, if I remember correctly, the game engine reads the libraries from top down, so you'll need to place your own libraries in the correct spot (after the default libraries ~ I think if I remember correctly) if you plan on using the default libraries along with your libraries (which would build upon, add to, or change aspects, of the default libraries).
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while it's a bit more complicated in how to code all your stuff in, the basic start method for creating a library is simply:
1. open up a new notepad, wordpad, or (best) notepad++
2. then just do ~ type in this:
<library>
(your mass of coding)
</library>
3. last is to save it, name it, make sure it is a quest file, " .aslx ", and make sure it is in the same place as your game file, that will be using it.
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here's where you can submit your own quest code (ie library files and instructions~explanations, or just directly as code too)
viewforum.php?f=18 (Libraries and Code Samples)
index.php -> Libraries and Code Samples
take a look in the links above, to see how others present their code stuff, to get an idea at how you should do it for your own.
enjoy