If you dual-class a thief to a fighter, can he use a longbow, composite bow, or crossbow after the he becomes fully dual-class?

I've only tried dualling a thief from a fighter and not the other way around, but I would say yes, they should be able to use all weapons available to the fighter.

No, they cannot use longbows, etc... The only weapons available are those that a theif can use. Although, they can wear any armor, they cant use theiving abilities with anything greater than studded leather.

I've done this and they can use any weapons availible to a fighter, and the only thieving ability you can use is Detect Traps. You can't disable them with armor on, but you can still detect them.

I played a dual classed fighter (6) - thief (8 or 9 from the start, 23 in the end). I took him over from BG-I ToSC where he whooped Sarevoks ass.
I still think that it was best to change from fighter to thief in the 6th level, because later you won´t get any special plusses, especially proficiency slots. The other plus of dualclassing is that a thief get´s quite a lot of proficiency slots - finally I had grandmastery in Katana AND Longswords.

When I imported my PC to BG-II SoA I was amazed because I was able to spend all my weapon slots wherever I wanted) without the restrictions of dual classing with a new fighter) so I got: Grandmastery in Katana, 2 slots for single-weapon style and the rest on longswords.
And best: You can backstab with a Katana - right from the start... ;)