When printing a file from okular, the printer selected by default is not the default printer, not the last printer used (actually, I don't get what is the criteria used to select the default, it doesn't seem alphabetical order either) In my humble opinion, it should the default printer, but the last selected printer would also be ok. Reproducible: Always
Unfortunately that dialog is populated by Qt and there's nothing we can do to alter its appearance. I would say to file a bug against Qt but Qt4 is effectively frozen for changes so there's not much luck there. Sorry about that.
No problem, it would be a waste of effort to fix something in Qt4 that is fixed in Qt5. One more reason for updating to Okular 5. Thanks!
For what is worth, the solution here http://unix.stackexchange.com/a/94587 fixed the problem: Removing /home/$USER/.cups/lpoptions