I actually did find a little more info yesterday after I posted, and of course had to test to find the real answer, and had not had the chance to post my findings yet.

 

In 9.0.1FP3, 'DisableDelKeyForNavigator=1' seems to allow mail to be deleted, but not the folder, which is exactly what I as an admin would want. 

 

However, I was hoping to find documented the EXACT intent of the parameter, because I'd hate to push it out to all my users, and find that in some versions of Notes that it prevents the deletion of mail from the inbox. Once pushed out, you can't remove notes.ini parms thru Policy (although one could set the parm to '0').

Re: your links - I could NOT find anything on 'DisableDelKeyForNavigator' here;

http://www-10.lotus.com/ldd/dominowiki.nsf/xpViewCategories.xsp?lookupName=Notes.ini%20File  - No results match the search query submitted.

or here:

http://www-10.lotus.com/ldd/fixlist.nsf/Notes.ini?OpenView   - No Documents Found for Query

or here:

http://www.lntoolbox.com/en/notesini-reference/bycategory.html  - Total: 0 results found.

Nor did I find it at the IBM support portal.

 

However, for the benefit of other users, I did find these:

http://www.pmooney.net/dload/ls14/BP101AdminblastFinal.pdf

http://www.idonotes.com/IdoNotes/idonotes.nsf/dx/ibm-notes-9-stops-user-from-deleting-folders-by-accident.htm

http://www.slideshare.net/MatNewman/bp104-have-it-your-way-amd-make-it-work-for-you

 

It would be nice, though, to find an official IBM document on this (or many other) subject(s).