Since this is the only link in the referenced thread that involved any file creation or code manipulation of any type;
It must be the solution you attempted that "crashed my magazine's website and I'll be down for several hours thanks to this".
I just followed the referenced instructions on a test installation of WordPress 3.4 Beta 4 using the twentyeleven theme. The only reference I changed was the category ID's. It worked exactly as stated. Then just for giggles, I applied four different custom permalink structures to the test site, and re-tested the custom feed with each one of them. Again, each time it worked as stated.
I suspect you probably made an error when you added the code to your themes functions.php file, which in turn, rendered your theme useless. I only say that, because all of the other steps are so... simple.
Don't know what else to say, other than I used the same info you did, and failed to experience the same level of failure as you.
I might be willing to try the same thing on a version of 3.3.2, if you like, just to be "experimentally precise". But I don't think it will make any difference.