IMPORTANT: the program you have linked is that of LAST YEAR! These sort of festival, with all its complexity (there are more than 200 activities, concerts, etc) don't publish the program until a few weeks before the venue, so you'll have to wait until the middle of Oct or so to see this year's program. The festival runs, this year, from Oct 27th to Nov 5th. This is the official website, so stay tuned: http://www2.girona.cat/ca/firesdesantnarcis
Anyhow,
1.) There are different displays of firework and firecrackers, but indeed "the" big one, called Castell de Focs (literally 'castle of fire' in Catalan) is on the last night of the festival, something very typical in Mediterranean festivals where fire plays an important role.
2.) It can be, if you have no other choice... but it's greatly advisable to spend the night there -or more than one if you can. Note that trains (and buses) to/from Barcelona are particularly busy these days so don't forget to pre-book!
3.) For God's sake: Girona "city".... unless you want to be stoned to death by Gironins, LOL!!!! It's nearly 100,000 pop.
4.) Plenty, this is a very romantic city, especially the Old City. Check out their tourism website. Note though that very often, as Ann mentions above, Girona is used as a base for many visitors as there are plenty of things to visit from there, from Besalú, to Figueres, to Costa Brava to the Catalan Pyrenees and a bunch more.
5.) It's crowded, from beginning to end, period, albeit during the working days there are fewer chances that many outsiders can pop up to visit. Still, we Catalans are very outgoing people and there are no festivals that don't draw crowds.
In any case, take my answers with a pinch of salt and wait to see this year's program.
Enjoy!