How do I change or remove default apps for a given file extension (in Terminal?) without munging date modified on all my old files?

Question

When a program is installed, it seems to be able to do it (sometimes inappropriately if it hijacks file associations without asking) so why can't I?  This is for 10.4.
Specifically:
1. Since installing an HP wireless printer, HP not only tries to install a bunch of sales-ware on your machine that you have to carefully opt out of (annoying, but not a problem if forewarned), it changes the default file association for a number of basic file types, listing all sort of existing files as HP printer files. Using Get Info to change the default file type association globally causes it to mark ALL the files of that type as newly updated, munging the file metdata (which HP did NOT do). Most of the stuff on our computer is sorted by last modified, so if there is a way to fix this in Terminal (to simply fix or remove the HP default tag) without changing the date the file was modified, it would help. Note that uninstalling the program that created the default would do this, but I obviously can't do that (and reinstalling HP would have the same effect). So there has to be another way, right?
2. Since updating iTunes to v.9 a while back, every mp3 is marked as an iTunes file even though iTunes has NO multiple library capability without a wonky reboot using the option key, meaning that if I click on an mp3 downloaded since last update of iTunes, it opens automatically in iTunes and gets added to the SOLE itunes library without asking. (There is no way to toggle between multiple libraries or archive media in separate folders; iPhoto has a similar problem. (Has this been fixed in the latest version of iTunes?) In comparison, Quicktime starts up immediately and does not attempt to create a playlist just because you opened a file and listened to it.
(The iTunes playlists are simply subfolders of the single main library -- and you can't create a new library without doing a wonky procedure by clicking option during startup and telling iTunes to forget where its library is located (includin everything in it) and look up an entirely separate one by hand. So any mp3 file downloaded recently is uploaded to iTunes if you click on it. This wouldn't be problem if it didn't automatically add it to the sole iTunes library just because you clicked on it.)
Besides, all my old mp3s are Quicktime files and they are all archived by date modified. To its credit, ITunes, unlike HP, does NOT hijack older files already on disk and remark them as iTunes files. It only marks newly downloaded files as such.  But if I try and change file association for FUTURE mp3 files using Get Info > "use this program for all files of this type", it munges the file metadata (date modified) for all existing files!
Note that in HP case, it apparently does it at the file exchange preference level (or whatever it's called) since the OS then recognizes every file of those types as an HP file. In the iTunes case, it only does it if you download a new media file or open an existing file in iTunes, but again without changing the date it was last edited.
Since these programs are able to change the default file association without changing the date the file was last edited, it should be reversible in Terminal, correct?  Why does it change "date modified" when you try and correct metadata using Get Info (or sometimes just by clicking an enclosing folder?) Date modified should be a record of when a file itself was edited, not when it was renamed, or other metadata. Otherwise stuff can't be sorted correctly.

Answer 1

The unix commands you need are:
GetFileInfo
SetFileInfo
and maybe find
for cryptic details use the man command
Macintosh-HD -> Applications -> Utilities -> Terminal
man SetFileInfo
You may use the SetFileInfo command to set the file type & the program which will open the file.
# This little gem will do a get info on all files in a directory.
mac $ ls  | xargs -I {} GetFileInfo "{}"
file: "/Users/mac/playdoc/oddadocodd"
type: ""
creator: ""
attributes: avbstclinmedz
created: 05/01/2011 14:53:22
modified: 05/01/2011 14:53:22
file: "/Users/mac/playdoc/one.docx"
type: ""
creator: ""
attributes: avbstclinmedz
created: 05/01/2011 13:57:48
modified: 05/01/2011 13:57:48
file: "/Users/mac/playdoc/oneLineFile"
type: "TEXT"
creator: "!Rch"
attributes: avbstclinmedz
created: 05/07/2011 14:27:17
modified: 05/07/2011 14:27:17
file: "/Users/mac/playdoc/oneLineFile.txt"
type: "TEXT"
creator: "!Rch"
attributes: avbstclinmedz
created: 05/07/2011 14:27:49
modified: 05/07/2011 14:27:49
file: "/Users/mac/playdoc/three.docx"
type: ""
creator: ""
attributes: avbstclinmedz
created: 05/01/2011 13:58:03
modified: 05/01/2011 13:58:03
file: "/Users/mac/playdoc/two.docx"
type: ""
creator: ""
attributes: avbstclinmedz
created: 05/01/2011 13:57:56
modified: 05/01/2011 13:57:56
file: "/Users/mac/playdoc/weirder.doc.docx"
type: ""
creator: ""
attributes: avbstclinmedz
created: 05/01/2011 14:50:03
modified: 05/01/2011 14:50:03
# well, ! is a funnie character so we escape it.
mac $ SetFile -t TEXT -c \!Rch two.docx
mac $ GetFileInfo two.docx
file: "/Users/mac/playdoc/two.docx"
type: "TEXT"
creator: "!Rch"
attributes: avbstclinmedz
created: 05/01/2011 13:57:56
modified: 05/01/2011 13:57:56
mac $
mac $ date
Sat May  7 14:40:56 EDT 2011
mac $