Itinerary help in Spain

Before you make any itinerary order you should figure out and settle on a price of the air fare first;
what cities you will fly in an out of. (e.g. fly into Malaga out of Barcelona).

Book the flight first before fares go up and then fill in the blanks.

OK. Using hopper to hone in on the cheapest flights. Round trip is the most economical way to go, but getting back to Malaya from Barcelona is awkward isnt it? I didnt see an easy way

If you wish to see Seville, Cordoba and Granada, but have booked a week at a Marbella resort, then your trip is seriously flawed. And then wishing to add both Madrid and Barcelona with only another 2 or 3 days, you are just compounding your problems.

Ok. I was afraid of that. I'll have to curb my wanderlust 😒

Go where you want to go logically to you.

Why Marbella? There's not a great deal to see there (I'm told) and it's miles from anywhere else, and no trains. Beaches are pretty deserted then. Are you planning to rent a car? It's an hour's drive just to Malaga or Ronda. At least another hour to Granada, Sevilla or Cordoba.

When in March? The last week of March is Semana Santa which is huge in Andalucia.

Well, I am not going to comment on Marbella, but as you have booked it, then I guess Malaga is the closest international airport. With only a few extra days, stick to the area. You can see Seville/Cordoba OR Granada/Cordoba. Otherwise, take a flight to Madrid.
One city with a day trip is about all you can manage unless you prefer to spend the time just on the train or plane.
As to cheaper round trip - for my location, I get no great deals and can do an open jaw trip for very little more than a return. Look at your options then add in the travel costs and time lost to get back to your original city. I find $200 an acceptable difference that allows me to go open jaw (depending on the countries or distance involved). At times it is false economy having a return flight when it causes a lot of hassles or in country expenses. I accept that it is easier for one person to adapt to open jaw (financially) than a family of 5, but I have seen a lot of planning frustration trying to get back to a starting point with costs, time and sometimes statutory holidays against them. Just be open minded.
Next visit look at just Madrid and/or Barcelona. A 3 hour fast train between the two.

Apologies for the reality check.
Now realistically, and with a commitment to Marbella for a week ...
•Fly in and out of Malaga.
•Possible day trips from Marbella to Ronda, pueblos blancos, Gibraltar, Malaga.
•Head to Granada for 2 nights and visit the Alhambra.
•Finish up with a night in Nerja before flying out.

I found a flight with a layover of 12 hours in Madrid. Might I sneak out of the airport for any sites?

I don't see why not.

I don't know about luggage-storage options at the airport itself, but there is a left-luggage office at the Atocha train station, which you can reach easily by multiple forms of public transportation. The three major art museums are in the same area of town as Atocha. Also nearby are El Retiro Park (free) and the Botanical Garden (modest fee). The Plaza Mayor, Palacio Real (entry fee), Mercado de San Miguel and other sights are also walkable. (I'm assuming no mobility issues.) Or there's public transportation, of course. I found the subway quite easy to use.

Check hours of the left-luggage office very, very carefully. There might be closures around meal times. And you might run into a line when you arrive to pick up your items, so allow plenty of time.

If there is a 12 hour layover, I don't need to deal with my luggage. And theres no further cost to get to Malaga
I "chose" marbella because it's a timeshare week exchange and the only one in Spain with 2 bedrooms to accommodate my family. It's either marbella or Montana 😁

We stayed near Marbella about 10 years ago on a family vacation and it was the best trip we've ever taken as a family. We went on a day trip to Tetouan. Took a day trip to Ronda. Went to a bull fight. And went on an open jeep "safari" type private tour inside a reserve. The guide was fantastic and took us into a little village (Istán) and to a goat farm where we milked the goats and ate cheese and talked to the farmers. It was so much fun. I'll see if my father has any of the info anymore if you're interested. All I remember is the guides name was Steve. There were plenty of day trips we could have taken from there but we didn't have enough time.

Thank you so much for a hopeful response. I started feeling incompetent about booking time in Marbella. Tour books like it but its not Madrid Or Barcelona, so it's nice to hear good words about your experience. Thank you!