Update
About the Wilco problem:
I've just confirmed that the 737 PIC installer does change the DLL.XML file, in fact, it changes it from "Dos/Windows" type (with CR and LF chars as linebreaks) to "Unix" type, with just LF. If you open your DLL.XML in Notepad after installing Wilco 737, you'll see the whole file appears like a single line. This because all Dos/Windows editors (unless they are Unix-aware) are expecting files having CR and LF to mark end of lines.
While this is not a problem for a full XML parser, because linebreaks shouldn't count and, in fact, IE is perfectly happy with it, this might create all sort of problems, especially if you try to edit the file by hand, to fix it yourself. It becomes very easy to make mistakes, when everything it's on a single line, to be manually splitted.
But the funny thing is:
Our installer, even the current version, is NOT confused by it!! I thought it might be, but in fact, I just tried installing the 737 PIC, then ran the Zurich installer, and there are no problems whatsoever, not only the installer is capable to add the Addon Manager lines, but it even FIX back the file to the correct Dos/Windows format, with CRLF as it supposed to be on a Windows machine!
So, why the issues ? Since I've verified that our installer doesn't get into problems with the file changed in that way, the only explaination I have, is that the DLL.XML was already wrong before installing Zurich, probably because it was hand-edited, maybe trying to fix the problems created by the 737, and since the file becomes almost unreadable as a one-liner, it's really easy to miss a closing tag when hand-editing such file.
That's the only explaination I have right now, unless we find other evidence of strangely formatted files.
Note, the installer doesn't really care what was in your DLL.XML file before: it simply reads the whole file, takes ALL lines (as they are) from after the header to the end, adds Addon Manager section just after the header, and write everything back to file. So, if you now see a missing closing tag in the middle of the file, the problem should have been there already, since the installer doesn't touch anything of the in-between lines, they are simply copied as they were.