Ah, sorry, but i don't have those bugs.
At home kdebase4_workspace is launched and two users of my PC launch there KDE3 session without any problem while i launch my KDE 4 sesssion.
If my proposition wasn't work i won't ever write it.
Perhaps you have a problem with your installation, since you can't launch a simple kdesu konqueror
For your kde3 users, in their home directories i think you can delete:
Check if .kde link to .kde3.5
Relaunch the kdm@KDE3 and login.
If problem persist, i don't know where is the problem.