Hi, As far as sexing goes, the drake makes a very low pitched whispery quack, and the duck makes a definate quack, the male usually has a curl on his tail as well.
She looks like a duck to me, and with those wing feathers she may have flown in as they arent clipped.
If you do release her with the wild ducks, she may find a friend though she may find a duck eating friend too!!!
Some people dont like to see domestic ducks with wild ones due to diseases, however I dont think the ducks care!!!It would be easier on you to rehome her to a duck family, they are very messy and make a real mess of your coop,.
My 2 duckies have their own paddock because they scared my chooks so much that they went off the lay, and they made lots of work for me changing the water every 5 minutes!!!
However if you do get attached to her and I would!!!get her a friend and fence them off with a paddle pool!!!
good luck
hugs