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I don't know how much you need to adjust the antenna width to compensate for the decrease in depth, but you can either do some wireless transmissions antenna calculations to determine that, or some trial and error?
You could have the ground plane on the same layer as the antenna, but the purpose of the ground plane is to provide the back wall of a monopole antenna. Moving the ground layer to the same layer prevents you placing it for a monopole arrangement, and prevents the noise reduction benefits of the ground plane wall behind the antenna. It does allow for a more omnidirectional antenna however.
Ultimately, I think you would need to test the layout you go with to find the best option, or to do some wireless design analysis at a more mathematical level to determine the affective results.