We have planned a overnighter to Morocco.
1. How long a drive from Seville to catch the ferries
2.Is there a place where we can leave the rent a car over night
This doesn't answer your question but l'd suggest looking at flying from Seville to Marrakech round trip on Ryanair.
- My guesstimate, and you do not say where you are aiming for in Morocco, but closest ferry departures being Algeciras, Tarifa or Gibraltar, about two hours. There are many web based route planners and google mapping.
- Yes, wherever.