Dual Blade weapon loadout questions

No, you can't use ANY DB skill without blade except Flash Bang and Owl Dead. Also, it doesn't multiply your range for 0.x to get dagger and blade damage. Dual bladers have 3 damage ranges: Dagger one, Blade one, and both combined. So, if you use a lvl 10 blade and then a lvl 90 one with te same dagger, your dagger hits will do the same damage, even though your damage range shown in stat window changed a lot. Only blade hits will be increased.

And sorry, I really thought we could equip shields, cause every job could.

The modifier is there to scale the dagger and blade ranges from your damage range, so it's the same thing.