Certified GKH Gardening Expert
No matter how much protection you give your key lime tree, it cannot survive the winter outdoors where you live. You are best off putting it in a container and bringing it in for the winter each year. I live in a similar climate as you and I have an orange tree that I treat this way. I have had it for 13 years now and it has grown happily outside in the summer and inside as a house plant in the winter. Your key lime should be fine this way as well.