Hello.
Answer 1) You can get a maximum of +12 from items and spells to any attribute.
Greater Rage should give you +6 to Con and Str. If you have a further +6 from other abilities or items, then you will cap at +12.
Answer 2) I think currently the shield brooch gives you +4 to ac from Shield, rather than deflection.
And thus, if you used a +2 large shield, and then switched to a +3... it still would count only the +4 from the brooch(Plus the 2 base ac of the shield, but only one shield ac enchantment can be in effect at once, and the shield brooch gives more than the shield's own enchantment). That is my guess.