#1: It is unclear what you exactly mean by "iTunes video". The video file you are talking about has the file extension *.mp4. MP4 is a container format, i.e., the extension alone does not tell you (or me) which codecs the file was actually encoded with (there are many possibilities). If the video uses some kind of DRM, it is very well possible that it cannot be played back in anything *but* iTunes. It may or may not be possible to convert it in this case. If conversion is an option, convert it to either WMV (for Windows compatibility) or MPEG-2.
#2: At the risk of stating the obvious: Merely *downloading* a codec pack (here: K-Lite) is not enough. The codec pack must also be *installed* on the system (i.e., execute the setup file you downloaded). It's unclear from "I've downloaded the K-lite pack now..." whether you've done that or not, hence I'm pointing it out for the sake of completeness.
#3: When you specify
/items = ("Video.mp4")
/playthrough = true
Inquisit expects to find a file called "Video.mp4" in the same location as the script. If the file is located somewhere else or called differently, Inquisit cannot open it.
#4: Finally, "it's still not letting me open my iTunes video via Inquisit" is also somewhat unclear. What exactly is the error message and have you made sure that the video is located where it's supposed to be (#3 abvove)?