I would certainly urge you to go to the Alhambra (but also implore you to drive carefully in the winding mountain roads - for the sake of others as much as for yourselves!).
I don't know where the bus you are thinking of taking would drop you off - it seems reasonable to me to assume that you could make the 11-11:30am slot with a bus scheduled to arrive at 11am... (roads shouldn't be that busy) but who knows? Perhaps it
would be best to take matters into your own hands and get a car.
I don't know Malaga and have never used the port - but I
can make a couple of phone calls (in Castillian Spanish) on your behalf and call the Port directly to find out which hire companies are located there. With that information it'll be quite easy for me to find a company who will have car keys ready and waiting for you.
I don't see why the Alhambra would be hugely crowded that day - tickets are limited to a controlled number and it's usual for it to be sold out. Whether it sells out a few weeks in advance or on the day itself won't make much difference once you're inside, the amount of people in there with you will be the same.
If you are serious about going, just let me know and I'll start calling. I think the Alhambra, Generalife and Granada itself are extraordinary places, I'll be delighted to have played my part in helping you see them.
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The other suggestion would be to email the Alhambra
http://www.alhambra.org/eng/index.as...ativo/contacto , explain that you have tickets and that the bus from the cruise ship should get you there at 11am but ask if they will allow a little flexibility in case there is a delay. There's only two of you, it's very likely that they will agree.