Howdy MG,
Sorry to hear of the troubles here. There were a couple of "can't open the file" re-opening issues that were addressed in 2.6.1 which should work to reopen the file you mentioned.
Addressing the MidiDev.cfg problem, that's bad that your old MidiDev.cfg got wiped out during the upgrade process. (I don't know that that's what the Threatfire issue was, but it may have been since the MidiDev.cfg is a configuration file).
Smart man to have a backup copy! (And it's always wise for any heavily modified files to keep a backup copy.) I'm not sure why that would have happened, because I did test for that problem here and didn't have any problems upgrading from 2.5.2 to 2.6. I'm testing on a Windows 7 machine here, though, and if I recall correctly, you're using XP? After restarting your machine, you should be able to copy/paste your backup copy of the MidiDev.cfg into c:\program files\Notation\Composer\ and have it correctly get used rather than reinitiated.
Do keep a backup copy handy, as well as make a "Device Description" file by using
1. Setup/MIDI Device Configuration (the big one)
2. Device/Export Device Description
3. In the left-hand panel, select the device you want to export (eg. your Creative Banks
4. In the right-hand select the device (if you've saved one of these before) or select "New Manufacturer" if this is the first time you've saved one.
5. Click the "Export" button in the middle of the dialog box, and then give the file a name and click "Ok."
Now if anything happens to your MidiDev.cfg file, you can always then use Device/Import Device Description and bring your bank information into the current MidiDev.cfg while running Composer, and avoid all that retyping.
ttfn,
Sherry