I don't think Granada is practical as a day trip from Seville. ViaMichelin estimates the driving time at over 3 hours without any stops, and folks say those estimates are usually optimistic and need to be bumped up a bit. I think you'll need to sleep at least one night in Granada.
I assume the reason for going to Granada is the Alhambra. I know from posts on this forum that it sells out early. I'm not sure whether April tickets are on sale yet, but if they are, you need to check on availability immediately. You're traveling during Easter Week, which is a very big deal in at least some parts of Andalucía. You may run into issues lining up hotels, too.
Edited to add: I took a look on Google Flights and for your dates (flying out of Washington 4/7 and out of Seville 4/16) I found a good number of options in the $1300 - $1500 range. Many require puddle-jumpers to a domestic gateway like Philly or NYC, which would not be my preference, especially when you have so little time to begin with. If something went wrong on that domestic flight, you might well be held up 24 hours. Given the Easter-week situation, it might even be worse. I would prefer to change planes in Europe on the way over.
The return options are not so attractive in most cases--much longer, or two stops, or separate tickets for the two legs (leaving you holding the bag, I assume, if the first flight is delayed).
You might take a look at returning from Madrid; perhaps the options would be a bit better. The AVE trains would get you from Seville to Madrid in 2-1/2 to 3 hours. You'd need to be in Madrid the night of April 15 to be safe. You can check out the train schedules and fares on the Renfe website. You'd be traveling from Sevilla-Santa Justa to Madrid-Puerta de Atocha. I don't know whether this link will work, but for April 15 I see that four of the trains are already full.
Because of the timing, I'm afraid some key aspects of this trip may be more costly than you're expecting, so I suggest that you shop for hotels and make cancelable reservations before buying your airline tickets. Best to know what you're getting in for.