Need help with itinerary for southern Spain

Check options for train or flight from BCN to Sevilla and Granada, then put Madrid last (both are easy train ride to Madrid).

As drawn, you'll likely end up taking the train to Madrid the night before your flight home, making the Granada/Seville order 6 or half-a-dozen.

You cant have bought train tickets yet, so you could fly from Barcelona to Sevilla, and work your way back to end your time in Madrid. That would have less back-tracking.

I agree with the others that you should spend your 4 days in Madrid at the end of the trip so you don't have to waste time backtracking.

I notice you don't have Cordoba on your itinerary. While some people visit Cordoba as a daytrip from Seville, I highly recommend spending a night or two in Cordoba. The city is beautiful at night when illuminated and much quieter after the day trippers have left. I would spend 4 nights in Seville, 2 nights in Granada, and a night or two in Cordoba, if you have the time.

I plan my trips in nights, not days. So when you say "days", do you actually mean "nights"? Don't forget that 2 nights essentially gives you one whole day, 3 nights gives you 2 whole days, etc.

The reason we are going to Madrid in the middle of the trip is that we are doing Rick's Barcelona and Madrid in 8 Days tour. We are flying home from Madrid because it was the least expensive route. So we can't stick all our days in Madrid at the end. Your suggestions to see Madrid at the end are great advice but unfortunately we can't.
Another question ... it was suggested that we include Cordoba, and we've thought about that. The tour includes a day -- but not a night -- in Toledo, and I've heard such good things about Toledo at night that we've thought about spending a night there after we leave Madrid before we go further south. Does that make sense? Would a day and a night in Cordoba be a good substitute?

Here's our full itinerary at this point:

Day 1 - arrive in Barcelona, not really counting this as a day
Day 2 - Barcelona, sleep there
Day 3 - Barcelona, sleep there
Day 4 - Barcelona - RS tour begins
Day 5 - Barcelona - tour
Day 6 - Barcelona - tour
Day 7- tour starts in Barcelona but moves to Madrid, sleep in Madrid
Day 8 - Madrid - tour
Day 9 - Madrid - tour
Day 10- travel with tour to Toledo, spend night in Madrid
Day 11- tour ends, spend rest of day and night in Madrid
Day 12- train from Madrid to Toledo, spend the night there
Day 13 - train to Granada, spend the night there
Day 14 - Granada, sleep there
Day 15- Granada, afternoon train to Seville, sleep there
Day 16- Seville, sleep there
Day 17- Seville, sleep there
Day 18- Seville, return to Madrid in afternoon for flight home the next day

That was pretty vital info, lol.
It really does not matter which you go to first then.
RE Toledo, just be aware you have to back track to Madrid (train to Toledo goes to and from Madrid).

Because you will be going to Toledo with the tour group, I personally would not return even though Toledo is lovely at night. For diversity, I would spend that night in Cordoba. Visually and architecturally, Cordoba is very different from Toledo. The Mezquita is an important historical and cultural site, but there is more to Cordoba than just the Mezquita. For example, the Alcazar has beautiful gardens, the Juderia is so atmospheric (with its white-washed walls, narrow lanes, and patios) for wandering and exploring, the Roman Bridge is beautiful at night. My vote is to spend that night in Cordoba instead of Toledo!

Regarding the order of the itinerary, you could go from Madrid to Cordoba, then on to Granada, then to Seville, and return to Madrid afterwards. It really depends on the train schedules and what works best for you.

As many (not all) of the trains run, Cordoba is on the way between Madrid and Seville, between Madrid and Granada, and between Seville and Granada. There aren't a lot of fast trains in and out of Granada, so it's good that Cordoba potentially fits into southern itineraries in multiple places.