Regarding will

If mother got the property through will, and wish to give to her son, she may execute a gift deed in favour of her son.  If she wants to give the property through a will,  her son will get  rights over the property, only after her death.

YES.IT IS CORRECT.  MOTHER CAN TRANSFER THE PROPERTY TO HER SON BY A GIFT DEED DURING HER LIFETIME. IF SHE WRITES A WILL, IT WILL BE EFFECTIVE AFTER HER DEATH

A Will takes effect after the death of the testator. Once the husband dies, the wife becomes the Widow. Yes, the widow can make will of the property beqeathed by her late husband