If I’m right your problem is that after a clean install of 11.3 (without formatting your /home) you have problems with kmail and some other applications.
I suggest you create a new user and copy over user data, no settings from your old user.
And then reconfigure the troublesome programs.
Although copying the config files should/could work it’s better to reconfigure to prevent such problems.
About upgrading your KDE I think you should go with what works and what you need.
Thank you for replying - I arrived at the same solution, but I ran into permissions problems on my email. Not wanting to risk screwing things up with a ham-fisted chown I put it to one side.
I went to KDE forums where the problem had been more generically identified as being akonadi with the following solution
Remove ~/.local/share/akonadi, ~/.config/akonadi and ~/.kde4/share/config/akonadi* outside KDE.
Solved everything
As for your advice, yes I agree entirely philosophically, but enjoy checking out KDE improvements. While I knew about the nvidia problems, I had never experienced them and I wasn’t expecting to trip up moving from 4.5.95 to 4.6.0