I have a relative who is a former Brinks guard. Pistol griped shotguns are to be avoided. You want a proper shotgun butt in the case you only had one are available to shoot (due to injury or any other reason)... You could prop the shotgun butt against a tree or wall, and still accurately fire.
For critical home defense, 8 shot 12 gauge would work well, as it does not go through walls and not a lot of accuracy required... However if an intruder had a family member at risk, accuracy of a hand gun is preferred.
As for bear protection... I'd suggest not to carry an 18" shotgun. As others mentioned, you really have no accuracy with slugs beyond 20 yards in such a short shotgun barrel (no rifling).
I would suggest a 45-70, something like the Marlin Model 1895G 45-70 Guide Gun Lever Rifle, 18" Blued, Walnut Stk... The rifle is very compact, and fires what is essentially a very long .45 ACP bullet which packs a tonne of power.
Bear defense requires protection beyond 50 yards. A 45-70 is accurate, and will stop a bear.
LOL, be sure to practice at the range with the 4570, they pack one heck of a kick.