Pickings are slim for Nerja. Europcar seems to be the only large rental company in Nerja that gets good reviews. Sixt has an office but it has mixed reviews on the forum. Europcar has offices at the train station and airport in Sevilla. Sixt also has a couple locations in Sevilla. As long as you’re going to Sevilla after Arcos, just return the car in Sevilla. From Arcos, take the CA5101 to the CA4011. CA stands for Cadiz. Once you cross into Sevilla Province, the road number changes to SE6201. SE stands for Sevilla. Follow the signs to Las Cabezas de San Juan. So far, all the roads will be 2 lane rural with little traffic. Prior to Las Cabezas, if you don’t want to pay a toll, pick up the A480. It pretty much parallels the interstate. If you don’t mind paying a toll, then pick up the AP4 / E5 at Las Cabezas de San Juan. Both roads will take you to Sevilla, but the toll road will be quicker. If going to Jerez de la Frontera, and have the time, the Alcazar there is nice to visit.
As a side note, if you have a GPS from the states, don’t be surprised if it says California 5101 for the road name rather than Cadiz 5101. From my trips, AL (Almeria) was Alabama, CO (Cordoba) was Colorado, AS (Asturias) was America Samoa. Once you figure out what the GPS is doing, it isn’t an issue. Also if you have a GPS, plug in parking lots in the towns and cities you’ll be visiting. By looking them up ahead of time, it will save you time trying to find them. In Arcos, park at the bottom of the hill (parking el paseo or parking plaza de Espana seem to be the closest) and take the tram up to the overlook. The road up to it is narrow and I don’t think there’s any parking up there any longer. The road down is narrow with a couple 90 degree turns. I’ve driven it numerous times and the last time It was very stressful because I had a larger rental car which made those 90 degree turns very difficult.
Hertz, Avis, and Enterprise all have locations at Malaga and Sevilla airports.