If 6mm uterus polyp is cancerous for my 56 yrs old mom and uterus is normal and ovaries as well then is removal of uterus the cure & ends cancer 100%?

No: Unfortunately, therapy for endometrial cancer is never 100% guaranteed. The risk depends on the exact pathology but luckily many of these patients (stage 1a) will not require further treatment or have a recurrence.

Depends: It is not possible to tell from your description. Consult with the md. If there is a cancer, a stage is given by the doctor. The lower the stage, in general, the greater the possibility of a cure. Stay positive and discuss with your mom and her md.