MK IV for defense
Glorfindel: since you have decided on a 22 lr semiauto pistol as most appropriate for your purposes, I would disregard all the comments about the superiority of other calibers & loadings as superior to the .22 as well-meaning but irrelevant opinions. We might all agree (but probably not) that 12 gauge 00 buck is (or #4, #1, et al.) most effective for in home use based upon terminal effect, overpenetration, danger to bystanders or neighbors, etc,. etc.
Read Bill DeShivs comments.
What disturbs me is the repeated claims that ALL 22 lr ammo is unreliable. In the past I participated in bullseye competition on a U.S. Navy team. Anyone who competes at high levels knows that very serious competitors will not tolerate ammunition wherein a failure to perform could cost them potential national or even international competitive ranking. I NEVER lost a point because of ammo failure. Even with using common CCI ammo, I have NEVER had a malfunction in my Beretta pistol, and over 30 years had ONE FTF in my Ruger MK II. In formal and casual shooting I have NEVER had a failure shooting Eley or Lapua 22 lr ammunition. If this is a concern, I suggest practice with the cheap stuff and use the more costly competitive-oriented ammunition for defense.
I believe your choice is very reaonable.