Yes, if you equip a shield the damage bonus in effect would be from the sword and shield expertise, rather than the one handed.
What are your relevant skill levels? If you've boosted Unarmed Expertise but not One Handed, that could explain the damage drop.
A 75% cap for Critical Chance seems likely. I tried a quick test just using the cheat window to get equipment and enchantment formula/ingredients, and got up to 73% duel wielding (the critical chance from the Butcher enchantment of each weapon applies to both).
