The last is the most logical answer to my ear. . .balance and stacking is an endless pain in the GM.
No reason the +10 bonus to the broadsword couldn't be explained purely as "It's mega magic sharp and hard!" and not a skill bonus.
Actually, that is a really good point actually, in terms of "What's skill and what's magical material strength?"
On the DB side, magic armor bonuses are implied to be material strength or some form of force field, since they affect all attacks, even ones the wearer is not aware of, and work even if the wearer is unconscious or 100% grappled, and thus lose their quickness/motion DB.