1. Called it1. Confirmed in today's chapter.
2. All the people that you mention use mainly their DFs to fight. Diamante is not a sword fighter he uses all kind of weapons he can create. So Basically for me they are all fighter depending on their DF.
2. Doflamingo uses his leg to fight against sanji, and clash with crocodile.
The ones that have devil fruits I understand, but despite the devil fruit people develope three cognitive fighting style that they can
Combine with their respective devil fruit...Right?
You can't just rely on your fruit...