I've always traveled through De Gaulle on my way to and from West Africa (used to take Air Afrique, but these days Air France, Ethiopian and Air Maroc all go to Bamako). However, since the patient wasn't symptomatic until they were in Dallas, transits and port-of-entry don't have to worry. There's a large, fairly dense immigrant neighborhood just east of the hospital in question (the IRC does a lot of refugee relocations there), so there may have been exposure in that area.

That would have to be somewhere in Europe.

There are flights from west Africa to both Atlanta and Houston, plus some other cities. United services Nigeria and Ghana out of Houston and Dulles respectively, Delta has some flights to the same cities out of their hubs.

It is most certainly possible to fly from west Africa to the US without transiting a third region.

ITA thinks there are routes from Monrovia to DFW which do not transit Europe. For example there is one via Casablanca and Montreal.