You are out of segments in Europe. You only get four with a max of two stopovers. There is no limit on how many times you can transit a city (as long as it is not your point of origin). Thus, GVA-LHR-DXB-LHR-DFW...LHR-GVA is the best you can do as you have used all four allowed segments.

In terms of mileage, you are only losing 36 miles by doing DXB-LHR-DFW rather than DXB-ZRH-DFW (8170 vs. 8206).

One more point, if you keep your transits of LHR <24 hours you will save substantially on taxes.