Most of your questions on the train schedule/tickets seem to have been answered.
We just returned from a little under 2 weeks in Spain with no pre-booked train tickets. I was able to navigate through the schedules and find tickets that worked for us while travelling, but we did have some flexibility in modifying our loosely planned itinerary. If I booked the tickets at least a day or two before, I usually got what I wanted. (Sunday was the trickiest to get tickets though). This was intentional as I was travelling with my 81 year old mother and needed flexibility in changing our schedule as needed.
Is your itinerary set in stone?
We would have loved to see Granada, but I ended up cutting it from our itinerary so we could have a little more time in other cities. Although there was a bit of regret, overall I am glad we did that. We also cut a day from Barcelona and from Sevilla. This enabled us to see Cordoba, Toledo and spend a day in Madrid.
It is obvious you are trying to cover the most territory you can in the short amount of time that is available to you. I have done this sort of travel and it can work, although exhausting. You also tend to shortchange yourself on some of the sights.
This is probably a far out recommendation, but would you consider this:
12/11 Madrid to Sevilla (train); night in Sevilla
12/12 Day trip to Granada; night in Sevilla - yes, this will be a long day; companies offer organized day trips there from Sevilla
12/13 Day in Sevilla; later afternoon/early evening train to Cordoba (only around 45 minutes); night in Cordoba
12/14 Cordoba to Barcelona (train) - there should be a 9:43am train that puts you into Barcelona around 2:30pm; night in Barcelona
12/15 Barcelona to Madrid (train)
Cordoba was easy to get around and did not require advance purchase of tickets to the Mosque. We also were able to see an Andalusian horse show on the evening of our arrival (purchased at the show). We only spent one night in Cordoba and it was sufficient for us.
You did not indicate if you will be spending any additional days in Madrid. If yes, you can plan to take an early train on the 11th out of Madrid and a later train on the 15th back to Madrid (to maximize time in Sevilla and Barcelona).