I'm not exactly clear from your post what is/isn't working:
1) You have 'lost' all your customised keycommands?
These are stored within the main Logic Preferences file. I think Logic 8 uses the same preferences file (perhaps you deleted that too?). Unless you have separately 'exported' them from within the keycommands window (such that you could re-import them) they would seem to have gone. Have you checked under Preferences:Keycommands:Options:Presets to see if you do hav ethem saved there?
If you have a Time Machine backup of your OS drive, you may be able to restore the old preferences file (<user>:Library:Preferences "com.apple.logic.pro.plist). If successful, you should export them straightaway as a backup.
2) Jampacks need to be re-installed?
Logic 9.1.8 "Download additional Content" assumes that all the jampack loop content must be in <Startup Disk> Library:Audio:Apple Loops:Apple. Are they in there, or have you previously installed or moved them elsewhere?
Are the JamPack loops available in Logic 9's loop browser?
1) Yes - I've lost my customised keycommands, AND many others just don't work eg. Go to next Marker ... these are greyed-out.
I located the com.apple.logic.pro.plist file and imported it. No change.
2) The Startup Disk> Library:Audio:Apple Loops:Apple. is empty. I checked User: Library:Audio:Apple Loops:Apple and they're listed there with an alias .... when I looked for them with Spotlight, I found them under iTunes Library .... is this correct ? I'm sure I never knowingly moved them anywhere !
3) Yes - the JamPack loops are available in Logic 9's loop browser.
In view of these anomalies, should I simply restore the whole Logic Pro folder from Time Machine (from just before I archived it ie the last version I was working on) to my Mac HD ?
1) It sounds like you may need to make a start by re-loading your default keycommand preset within the Keycommands:Option:Preset menu.
This should at least give you a (more than basic) set to check that all is well with the rest of the setup.
2) The iTunes library is not the "correct" place, but these Loops can live wherever you like as long as Logic's loop browser knows where they are (it does, if you can find and use them as is).
I'm not convinced that you need to restore everything just yet.
Thank you for helping me !
From where should I import the default keycommand preset ? I followed the Keycommands:Option:Preset menu path and was given a window with alternatives in French/German/Spanish.
I also clicked Import Presets and another window flashed up obviously inviting me to search the Mac.