I haven't read the whole thread, so if I'm being redundant, well, now you know why
My first suggestion would be to spend some time in the S&T section, and become familiar with situational awareness, avoidance, and the other "soft skills" of personal protection. The best gunfight is the one you never have to be in.
There are guns that are not much bigger, but in a more reliable and effective round. The Ruger LCP, Taurus TCP, and Kel-Tec P3AT come to mind. You're right that sometimes the presence of a gun is a deterrent, and that, in some situations, may be enough to get you home alive. When you present your firearm, wouldn't you want the aggressor to be looking down the biggest possible hole? There's something to be said for the difference between less than 1/4 of an inch, and almost half an inch in diameter.
I think a lot of folks have already discussed the advantages of stopping power, and the greater effectiveness and reliability of .380, 9mm, and the like, so I won't go into that too much.
I do wish there were a true pocket 9mm, as it appears to me that .380 ammo is even more expensive and I like to do a lot of shooting/training. My current plan is to do most regular training-target shooting with my .22 but also to acquire a pocket .380 for critical training and carry, in the near future.
You mentioned that the .22 lets you practice a lot more than a center-fire gun. By all means, keep the .22, and use it at the range to hone your skills. By having a small gun like that, you'll be better prepared when/if you move into something a little bigger. One of the moderators, (Sam1911?) has a sig-line that includes the progression: Mindset-Skillset-Toolset. THR is a great place to learn about the mindset* side of things, and while not the best choice for a defensive tool, your .22 would be a good way to develop the skill-sets needed for staying safe.
As far as skillset goes, I'd look into some of your local ranges, and see if any offer any kind of "fun-steel" or "steel challenge" competition where you could use a .22. When you start shooting matches, its amazing the kind of things you can learn from people. They may also offer some sort of training class, that may be a great step for you to take.
This is something I posted in a different thread a while ago, but I think it still applies, and is a good potential starting place :
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.
Hope this helps, and welcome to the High Road,
Chris "the Kayak-Man" Johnson