Lisa,
Best bet for simplicity ,at the moment, is to fly AA DFW-LHR and connect to BA6984 down to AGP ... BA flight is wide open at the moment ( you might make the BA6982 if the AA is very early but I wouldn't rely on it ). Use Gatwick if you have to but be aware that the coach transfer to LHR is about USD 40 pp. Arriving into LHR and transferring to Europe means that you pay the lowest rate of UK departure tax ... USD 20 pp ( rather than USD 80 ).
I would avoid the UK ( if you can ) on the return as you'd be paying the higher departure tax to get to the US.
Best bet would be to look at the loads on your flights into the US from Europe and see which has the best availability and then see how to get there from AGP.... you can get to most European capitals from AGP.
Also be aware that the 5th is on the weekend when the loads are generally higher ... if you can afford the time it might be just as cheap to stay an extra day or two and then return back after the weekend on a more direct routing ... say AGP-CDG-DFW for example.
Let me know if you need any more info.

P.S. Knowing Spains reputation ... make sure that the timeshare is actually built before you leave !!!