Yeah, I think that's telltale of the fact that they didn't really want to do that extra Keen game for the company they started out with.
I found most of the games:
Crystal Caves 1-3, Keen Dreams
http://www.studieboekentoko.nl/oldgames.html
(also has Commander Keen 1-6)
Secret Agent 1-3 I "found" on this site; but I have yet to find out how to actually download them. After clicking a title I could go to a page with "download" in its URL by clicking the word "TUTAJ", but fuck if I know what to click from there on. Damned Swahili websites.
http://klasyka.gery.pl/gry.php?strona=0&litera=S
When I went to Home of the Underdogs I at first found only Biomenace 1, then when I tried to find the others the site's speed went to shit. I found the games elsewhere, but be warned: for some reason, this site seriously sucks up your system resources.
Bio Menace 1:
http://www.emuarena.it/Files/SchedaA...ABELLA=ABANDON
Bio Menace 2:
http://www.emuarena.it/Files/schedaa...ABELLA=ABANDON
Bio Menace 3:
http://www.emuarena.it/files/schedaa...ABELLA=ABANDON