Have you checked the fares nonstop LIS - CMN on Regional Air (FN) yet? (Mo, We, Fr, Su at this time, I believe.) Or Royal Air Maroc (AT,) every day except Wednesdays?

Using IB and MAD, you will arrive T4 (Portugal is Schengen,) and have to proceed to T4-S for the flight to CMN (UA arrives / departs T4-S.)

Lisbon is a very easy city to get around - a wonderful walking city with lots to see. MAD is somewhat less so, and the airport is a significant distance from town (though the new Metro station for T4 makes it easier.) Just my preference, but I think LIS is less expensive and offers more for the visitor.