Planning a LHR-CUN (Cancun)
then CUN-YVR and YVR-LHR trip (involves a wedding in Mexico hence the small detour). Can anyone advise how miles would be calculated. Is it all taken as single segments or could it be a return to a midpoint eg Phoenix with an outward via CUN and return via YVR?

Is First on US charged at x2 or x1.5 as I'm guessing it's not really First but business class.

Much appreciated thanks!