Baldur's Gate : The Sword Coast Chronicles : NWN2 Persistent World

Duelist gets Deflect Arrows free at 9th level, but Deflect Arrows only works if you're unarmed as far as I know, unless it's changed here. Or does it work if you have one hand free? The feat description says "You can't be holding anything in your hands".

If it does work if you have a weapon in one hand and nothing in the off hand, that's fine. But if it doesn't then wouldn't Duelist be better off getting Evasion instead? Anyone know whether it works with only one hand free or not?

As long as your offhand is free, it works. Deflect Arrows requires either empty mainhand or empty offhand.

Does this include longsword, which defaults to use two hands when you have no shield?

Most similar mechanics only care about your offhand slot being empty. You could probably even use a greatsword and it'd be fine.