I went and looked for the crossbow segment of Helm's Deep battle and have to say...working with only a few seconds of footage(8:26 in the video), my friend, that's dedication!
You did get really close, they did use a recurve prod on the originals, but I think your Orc-Tech Prod is more authentic to the Orc ways.
After watching the video I also noticed the arrow strike sideways and watched it again more closely. No problem there, you just rested one of the limbs on the bench and that's what caused the problem...But that's what happens when you are in are in super sonic mode.
Nice build and brutal in all it's glory.
If I may add something to make it a bit better...
In the above picture I see a bit of a problem...the sharp angle right after the side plates is probably destroying the string right now...you definitely got to file/sand it under a slant so it's even with the side plate and there is no gap for the string to catch on as it travels down the track.
Definitely make that new string, perhaps a thick black bow string (some natural fiber like hemp dyed black) would give this bow the beastly Orc siege look it deserves. Ivo