April in Andalucia and points north

Very reasonable. I like that you are limiting your focus and getting to see some smaller cities. It may not be the most efficient route, but if you have to work around specific schedule dates, then at least the backtracking isn't major distances.

One thing to consider is skipping the area around Madrid this time and save it for 2016 when you will be back. Toledo, etc... are all easy cities to reach from Madrid. Perhaps instead you could add Valencia and some other less-popular cities/towns to the south. Or take the ferry over to Morocco for a couple days. Just something to think about.

Winging it is probably not a good idea during the Holy season in Spain and for certain sites and events (soccer matches, Alhambra). And you may know that RENFE offers very deep discounts on train tickets purchased in advance (use the Paypal option on the website).

Sounds very reasonable. It's only an hour by train from Jerez to Sevilla. It seems to me that you could go to the game from Jerez by train. Drive from Ronda to Jerez (and return the car there), visiting some of the pueblos blancos on the way. Also, is it worth 2 one-nighters (at the end of your trip, when you are likely to be less energetic :-) to stay overnight in Segovia and Avila? They can also be day-trips from Madrid. Another alternative is to rent a car. I've been looking at Rick's Madrid & Toledo book and there are some things that you'd be able to see and do by car but not by bus/train. Brad wrote a trip report in 2012 which may help you too https://community.ricksteves.com/travel-forum/trip-reports/spain-clean-toilets-affordable-lodging

Granada I went to the Nasrid Palaces at night, then back on the morrow to spend a full day visit. Its' what I would recommend. The Palaces are completely different after dark. If you buy a morning ticket with an entry after 11.30 a.m. you can get into the other ticketed areas of the Alhambra any time during the day.

It's more efficient to stop to see the flamingos on the way from Granada to Ronda.

Jerez Check which days the horse shows are (Escuela Ecuestre ). When I was there in February, it was only on Thursday. Well worth seeing. You will probably need to buy tickets in advance - a good idea anyway because if there are tickets available, the line can be really long. Get there early and you can see the horses up close before the show. Along with the sherry, Jerez is the flamenco capital. If you don't get there by car, Arcos is an easy day-trip by bus.

Cordoba The Royal Horse Stables have an evening horse show.

Thank you Chani and Douglas. Chani, your posts have been most helpful. I know about the flamingos enroute from Grananda, about checking what days the horse shows run etc. Great info! Good point about not being energetic at the end of the trip, I may rethink this. The transport legs are all fairly short at the end, so was thinking a 1 day stop wouldn't be too onerous, but perhaps we will skip Avila in favour of an extra night (and porchetta!) in Segovia. Will also look into car rental for that segment. Thanks again to you both!

Here's my 2 cents:
1-Avila: I agree that you could skip doing an overnight here. The city walls are beautiful as well as the historic center, but you could probably see all you needed in a few hours.
2-Granada: Absolutely agree w/ the earlier poster re: seeing the Alhambra at night. I did that & found it WAY better than seeing it during the day when it was packed with tourists.
3-Cadiz: IMHO, I don't think Cadiz has that much to see. I agree that Arcos is a beautiful hill town as well as another- Vejer de la Frontera. Perhaps consider these instead of Cadiz.

You've really planned a good itinerary, I'm sure you'll have a fabulous trip!

For travel within Spain, a good site I have found is goeuro.com. It
is easy to use and gives you info on air, bus, train and car travel times and costs between each place, all on the same page, so no flipping back and forth between sites. It also gives you the scheduled times for bus, trains and flights. Also will tell you if some of your trains will require reservations (most important for overnight trains).

I quite like Cadiz, perhaps Europe's oldest inhabited city.
I would suggest basing yourself in Madrid and do Segovia and Avila as day trips.

Thanks again for the suggestions. Revised itinerary is this:
Malaga 2
Nerja 2
Granada 2 (pick up car on way out)
Ronda 2
Sevilla 3 (drop off car on way in)
Jerez 2 (may do 1 night in Sanlucar de Barrameda)
Cadiz 2 (my husband works here from time to time and loves it.)
Cordoba 2
Toledo 2
Madrid 3 (base for day trips to Segovia and Avila)

First 11 days of accomodation booked, and will wing it from there. Through searches on this website I have found awesome info on car rental and how to drop car at Sevilla Santa Justa; planning time at the Alhambra; navigating RENFE etc etc. We are looking forward to an amazing self guided trip!

Don't forget April Fair in Sevilla (I believe April 21-26 in 2015). It's definitely worth including in your plans even if you have to make some adjustments.

I don't know how Semana Santa will be in Malaga, hopefully great. It's another great event. We spent most of the week in Barcelona and learned the hard way that, while Barcelona is great, it's about the least Catholic city in the least Catholic part of the country - so there was virtually nothing going on (except extra masses in the Cathedral) to celebrate the week. We spent Holy Thursday in Zaragosa which will always be one of the all-time great travel experiences of my life.

Here's the trip report I did for our trip:

https://community.ricksteves.com/travel-forum/trip-reports/spain-clean-toilets-affordable-lodging

Wondering if the car may be of use in and around Jerez and Cadiz.
Perhaps you should do these before dropping it off in Sevilla.
Just a thought.