1.) I'm not sure, but if you look in the appendices at the back of the Return
of the King, there is a lengthy segment on the history of Rohan, which is
probably the best place to look for something like that.
2.) I believe that there are more people to defend the keep in the book (so
they don't need the elves), there's also a dike which they try to defend for a
while, and a couple calvary charges and things like that).
3.)There are several Uruk-Hai arbilisters (arbilists are beefy crossbows) which
may be seen in the battle of Helm's Deep. However, since for the most part,
long bows have a range advantage over crossbows (except for the most powerful
ones) they would probably be more useful for defending a castle due to their
longer range (although you would experience a lack of firing speed, unless you
were an elf who has had 2000 years to practice his art, like Legolas).
Sorry about being
longwinded.
