What does platsy myectomy do? Does it reduce spasms? Lift?

Eye closure and your platysma muscle are completely unrelated.  Synkinesis often involves orbicularis eyelid muscles, innervated by zygomatic branches of facial nerve.  Cervical  branch innervates platysma, and cutting that muscle will influence only neck bands and spasms manifested in neck, not eyelids.

The platysma is a muscle that extends from your collar bone to your lower face. It controls facial movement in that area. As we age, it can loosen and start to sag resulting in bands that appear on the neck. They are sometimes surgically cut or trimmed and sutured to improve the neck contour for facial rejuvenation. Based on your question, I suggest meeting with a board certified plastic surgeon to discuss your specific concerns. I wish you the best.

I think there has been some confusion in terms....the platysma is a muscle in the neck and treated in various ways during a facelift to improve neck contour, but has no effect on the eyes. It sounds like you would do best to get an evaluation by an ophthalmologist (eye specialist) to go over your problems and see what is recommended to help.