1. I assume it's much like deer anatomy.
I don't know much about bear anatomy myself, so I'd guess you were right as far as layout; but I bet it is much different in terms of bone density, muscle, etc. A black bear can reach 500lbs, so it is going to need a heavier skeletal structure to support that weight, bigger muscles to move the skeleton, etc. Both of those probably mean that what penetrates fine on a deer probably won't penetrate as well on a bear. Of course, the key word there is "guess." I don't know much about the subject.
2. Honestly I'm not sure yet because I've never had to shoot under an extremely stressful situation. I'm going to guess my shot placement would be less than perfect, in which case 12ga slugs would be a better idea.
Well, let me give an example I am familiar with. If you are facing me with your arms down at your sides, then any round that can penetrate 5" is capable of reaching your vitals and I have a nice 8" wide or so target zone facing me.
Now raise your arms up above your head and turn sideways - oops, smaller target zone now and it looks different from this angle. Plus I've got a little more meat to penetrate so now 5" will probably get there; but I stand a chance of not having enough penetration.
Now drop your arms to your sides - uh oh, the amount of exposed area where 5" penetration is sufficient just got really, really small. I either need deeper penetration, or I need to be really good.
These same principles apply to a bear, except the bear may be charging you at speeds that no human being could achieve, so you'll have a lot less time to sort all of that out. I certainly think it is possible to kill a bear with .223; but if I was
defending myself against a bear, I don't think I would want to limit myself to those odds. Still, .223 beats sharp pointy stick.