I really wouldn't do Seville to Granada return in a day by bus.
The buses are generally fine, but the heat (except late Oct to March) and 3 hrs each way would do it for me. Also there are other things to see in Granada such as the Cathedral with the tombs of Ferdinand and Isabella and the Manuel Lorca Museum.
From memory, Granada bus station is a ways from the Alhambra, which in itself is a 400m stiff climb up from the old centre.
Furthermore, if you go at a busy time of year, you should book tickets in advance and you are allotted a time when you can go in.
You should check this out more on the Granada forum.
You can see the relevant page on the Granada City official tourism website here.
http://www.granadatur.com/en/city-guide/monuments/alhambra/how-to-get-tickets/
