When I went to Alhambra one October day I just walked up to ticket window - would you even need resrvations for your dates?

Yes, she would. Last time you visited was during the Dark Ages, before all this reservation business began.

Definitely book when the calendar is available! When I went 2 years ago, it didn't become available for my dates until just over a month before I planned my visit. It's so arbitrary, tho - sometimes it's several months before. Just a good idea to check regularly till it opens up.

kja - for any train to or from Granada, it is one ticket purchased from RENFE train company. You go to the train station in Granada and yes, they put you onto a bus. It goes to Antequera and they shephard you and your luggage over to the train. Load you up and to then go on to Madrid, Seville, Barcelona, wherever.
Yes, it might be disconcerting to find part of your trip on wheels rather than a track, but it is seamless and Renfe takes care of it all.
It does slow down the travel a little - to the extent that travel from Seville - Granada (or reverse), takes the same time for train or bus. As stated, buses in Spain are comfortable so either is an option. Train is still faster to Madrid, however.
Eventually it will be completed, but given all the broken deadlines, it could be years yet.

Trains v buses in general - Spain,where once it was said that 'the pain was mainly on the train', Spain now has one of Europe's most advanced fastest comfy rail systems with AVE trains rolling along at nearly 200 mph - so on high-speed lines much faster than buses and also oh so so more comfy -you can easily get up and stroll around -go to cafe car for a drink or snack -easily get to WCs - if a choice between train and bus and the trip is more than a few hours go for the train IMO!