I couldn't quite follow your logic, but this is how barkskin works.
- If your AC is less than 16, it is now 16.
- If your AC is greater than 16 it is not changed.
- If your AC was less than 16 before you cast barkskin, so it is currently 16, and then something changes to improve your AC further, then you calculate your AC with the new item ignoring barkskin. If the new AC is still less than 16, it's now 16. If the new AC is greater than 16, barkskin has no effect and you can stop concentrating on it.
- If your AC was higher or equal to 16 before casting barkskin, but then something happens which causes your AC to go below 16 while the spell is still active, your AC is 16.
What this means is that other factors are more relevant than the thickness of your skin, once you go above an AC of 16. To explain further, your skin is your "last line of defense". If you have a shield and armor, and those combined do not yet equal 16, then an attack that gets past your shield and armor gets to your skin, which because of the spell is a 16. However, your skin does not increase your effectiveness in using your shield and armor, so if your AC before the spell is greater than 16, your skin offers no extra protection beyond this. Since AC is an abstract concept which involves your many different ways of defending yourself, that which protects you best is the defining characteristic of your AC.
Being that this is a second level spell only available to Druids and Rangers and Nature Domain Clerics, the odds of having a dex of 20 to get a +5 to your dex modifier is very low, for at least another 5 levels. (This spell only becomes available at 3rd level)
Yes, you can use Unarmoured Defense in nonhuman form, and yes, you get an additional +1 AC on top of it.
The lycanthrope PC box lists a bunch of things that you gain; it's very different to Wild Shape, where the new form replaces almost everything about you.
A character who becomes a lycanthrope retains his or her statistics except as specified by the lycanthrope type. The character gains the lycanthrope's speeds in nonhuman form, damage immunities, traits, and actions that don't involve equipment. The character can't speak while in animal form.
So becoming a lycanthrope doesn't prevent the character from using any of their class features, including Unarmoured Defense. Now for the shapeshift ability, where you've already found the key line.
Its statistics, other than its size and AC, are the same in each form.
All that this is saying is that changing form doesn't change anything about you except size and AC. The size change is straightforward, obviously, but the exception for AC doesn't make much sense. That is, until we know that the werebear gains a bonus to its AC while in some of its forms. If shapeshift didn't make an exception for AC, you'd have this weird situation where the werebear was meant to gain +1 AC but at the same time wasn't allowed. So this line of text isn't forcing you to use the default AC of a werebear, it's just allowing for the possibility that changing form might change your AC.
It should be pointed out at this point that the werebear statblock is a default stablock for an NPC werebear. The sidebar on player lycanthrope is very specific about what changes are made to your character, and it doesn't say anything about using the AC from the NPC statblock. Note also that the AC of the werebear statblock (indeed, all the lycanthrope statblocks) is just 10 + Dex mod (+1 in nonhuman form).
So yes, you can use Unarmoured Defense in lycanthrope form. Unarmoured Defense says that it doesn't work if you wear armour, but turning into an animal is not the same thing as wearing armour.
Finally, the natural armour bonus. This is a bonus, so it can work on top of an AC calculation; it's not an alternative AC calculation that won't stack. You might be concerned that it's a "natural armour" bonus and would therefore invalidate Unarmoured Defense, but as above, Unarmoured Defense only says that it doesn't work if you wear armour, which having a "natural armour" bonus is not the same as.
Best Answer
Dexterity, Constitution, and Wisdom can be added permanently to AC. Intelligence can be added for a minute. Charisma can be added through a magic item from Hoard of the Dragon Queen or to a single attack. I found no way at all to add Strength to AC.
Below I will provide "the best" methods of adding a given ability modifier to one's Armor Class. By "best" I mean the one that lasts the longest, which, in the case of Charisma, is quite short. This means that features which add Constitution to AC for 1 attack, turn, round, minute, or anything else will not be mentioned/listed:
Barbarian's Unarmored Defense feature (Con and Dex):
Monk's Unarmored Defense feature (Wis and Dex):
Bladesinging Wizard's Bladesong feature (Int):
Unearthed Arcana's Oath of Heroism Paladin's Glorious Defense feature (Cha):
An item from Hoard of the Dragon Queen (Cha):
Thanks to user David Coffron for pointing out this item!