Good post, Old Marksman.
I'd add the following:
1) A .22 that the shooter can handle and is willing to carry is much better than nothing (and old age, arthritis, small stature can potentially limit somebody to a .22, based on hand strength).
2) However, a round that has better penetration, more reliable ignition (IE centerfire), and a good probability of breaking bone, if it hits large bone, is better - assuming one can carry and control it.
3) Learning to "shoot to stop" will get people to quit fixating on round count, per se; it takes what it takes, under the circumstances in which it occurs; this could be one shot, or it could be a dozen.
4) Given 3) more capacity is never a bad thing.
5) Something I almost never see brought up, when people talk about carrying spare ammunition, is that it might be nice to be able to reload, after the encounter, for the period between the shooting and the arrival of the police. The BG might have friends.