We have many Hadeeths about narcissus النرجس. In Tibb Al-Ridha , page 38, from Imam Al-Ridha : Don't delay smelling the narcissus, as it prevents cold in the winter.
Yes, it is allowed, although I am not aware of any traditions recommending it.
However, I think it is better not to use plastic flowers (which you see in some places instead of real flowers) for environmental reasons.
We do not have any Hadeeth or narration or practice of Ulama to place flowers on the grave. It is a habit from non Muslims. It can be waste of money. The deceased will get no benefit from it. It is good to pay the cost of the flowers to poor and needy on behalf of the deceased.