Answer: It will not hurt the plant at all to trim it back as much as needed to fit it indoors. Once inside, you can try to keep it in active growth by giving it a very bright location or you can allow it to rest during the winter. To do that, give it a cooler room temperature of about 45 degrees, stop fertilizing and water less; the soil should be just barely damp. It will probably defoliate in protest of the lower light levels when you move it inside, but it should survive.
You can propagate mandevilla with tip cuttings taken during the summer; you may want to use a rooting hormone (sold at garden centers) and provide warm soil and air temperatures with high humidity. You should be able to make multiple cuttings per stem, however make sure to keep them right side up with the original growing end pointed up and the original root end oriented down. It is sometimes difficult to succeed with cuttings in your climate because the growing season is so short but it is worth a try. If you have access to a greenhouse that would be preferable.
Good luck with your project!
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