Is it okay to wear prescription sunglasses over contacts?

Yes. But should not be necessary if the lenses are made correctly.
Yes....but... Why would you need to! If the contacts are the right power, you should not need glasses. Unless you have a very high prescription that contacts cannot correct.
Not really. If you have contacts on and add prescription sunglasses from the same prescription, you'll be doubling the power of the correction and likely be very blurry.