Mindset>Skillset>Toolset.
If I had to use a .22, I'd put a lot of work into the mindset and skillset before I started thinking about the ammo. Since you're asking this question, and you take the time the practice, you probably have the Mindset, and it sounds like your skillset is pretty good too. When I was reading your post, the one thing that stood out was that you were shooting from the bench. I'd practise as much as I could shooting from different positions and distances. Since .22 ammo is less expensive than most rounds, you should be able to get a good deal of practice in.
If it was me, I would strive to become proficient in:
-clearing manfunctions
-point shooting/indexing/instinctive/ whatever you want to call fast close range shooting
-shooting smaller targets at a longer than average distance (greater than 25 yards)
- engaging multiple targets.
-engaging multiple targets at varying distances.
-reloading (Especially since the Walther has that [to me] funky mag release in the trigger guard)
-clearing malfunctions.
Your own list of skills that need work may be different, but that is what I feel is important when talking about using a .22 in a gunfight.
As far as ammo choices go, I'd look for reliability, then penetration, then expansion. I don't care if the ammo expands to several inches in diameter if it doesn't penetrate enough to actually do anything, and ballistics are a moot point if your gun doesn't go bang.
I'd also look into using a .22 magnum, if that is at all possible. I think Kel Tec makes one called the PMR-30, that might suit your needs quite nicely.
http://www.keltecweapons.com/our-guns/pistols/pmr-30/
Hope this helps some,
And you're his son. I'd hate to see what he'd do to someone who isn't family
Chris "the Kayak-Man" Johnson