If I designed the duelist pistol and I dismantle it later, will I get all materials back?

Yes. If you craft something and then later dismantle it, you get all the steel and other materials back. So you can craft duelist designs using rare materials, and later use those materials when upgrading to demon hunter gear by dismantling them. Every duelist rare material requirement corresponds with the demon hunter gear of the same type. For example, the rare material for the duelist pistol can be used when you're ready to craft the demon hunter crossbow.