While planning a trip from Spain to Morocco, it might be very difficult to find the info  needed. To  travel from Seville to Tangier, take the Bus at Prado San Sebastien station. If you plan on traveling to Tarifa on a Friday evening, book your ticket in advance on www.Movelia.es.    If  the 5:30 pm bus is fully booked you have an option of taking the next bus at 8 pm which brings you to Tarifa at 23:00. Apparently, Cadiz is a popular weekend destination and the bus stops at the city before reaching Tarifa. If you prefer not to take the late bus and spend the night there, there is another bus company (Linesur) in the same bus station that leaves at 18:30 to Algeciras and arrives there at 21:00. Once there, the bus makes a stop at the puerto (the port) before continuing to the bus station. Descend there and cross the street to the other side. It is not a short walk to the port so pack light. After you buy your ticket (23 euros) check which hall you are departing from. There are several companies. There are ferries departing at 21:00, 22:00, 23:00, 00.00, 6:00, 7:00,... The ferry is supposed to take 90 min but takes about 120 min. On the ferry, fill a custom form that you can find at the bar and clear customs with the Moroccan officer. You will know he is there when you see lines forming behind the bar. Once in Tangier, you will have to show your passport again to the officer at the exit. The ferry arrives at (Tanger Med). A bus will come and pick you up with others to the Gare Maritime where you will put your luggage on the belt to be checked. There is an hour difference in the winter between both countries. Morocco is an hour earlier. Once out, a bus will pick you up to take you to the city, about 45 min. This is a lot of time.
Having done that,  it was wise  to change plans for the return. In the city, just outside of the city port, you can buy a ticket (32 euros) on the fast ferry FRS. You can book online at (www.frs.es) but no need to do so. The trip is only 45 min and departs from the city (Tanger city) so there is no need to do the 45 min road trip. You again clear customs and board. The ferry arrives to Tarifa which seems much nicer than Algeciras. If you are tight on time, you need to be at the front as once the gate of the ferry opens, people run to get to the customs. The lines are quite long. Once you clear customs, your options are as follows: take a taxi (7 euros) to the Comes bus station for the 17:00 bus to Sevilla (the last from Tarifa) or if you are late, take the free shuttle FRS provides to Algeciras. Use the word bus not shuttle. There, there are 2 buses back to Sevilla, one at 19:00 and the other at 21:00. The bus from Tarifa makes few stops on the way but avoids you the trip to Algeciras. 
Have more time to enjoy rather than worry. Good luck.