First off, I might be able to shed some light on some of the items which have already made their way into the most recent version (1.8.5).
preventDefault() is implemented for those keystrokes when the suggestion list is visible (same way you do it).
The bug with minLength has been fixed, and if the search term does not change, a search is not performed.
mouseover no longer causes menu item to show up in the input box.
In regards to the "submit" event, I see that you only fire it if the menu is not active. I wonder if that could be accomplished by attaching a separate listener to the input box. (More on the reason behind that later.)
In regards to the the maxWidth and fixedWidth options. Fixed width may already be achievable through CSS overrides... however maxWidth would be difficult in CSS in the cross-browser world, providing a valid use-case for the option(s).
And then we come to item.html... I think there is great use for this (I ran into a need myself not too long ago), but this brings us to "extensions".
Before I get into that, let me say that I have not taken an in-depth look at your code, so none of the above comments are meant to diminish the need for the modifications you made or as judgment to the way you accomplished them. The questions posed are simply to provoke thoughts towards alternative ways to accomplish the same thing.
You'll note that Scott's blog post, and my github depot both have different approaches to rendering html for suggestion items, so they may either solve your need or serve as an example in how you can reimplement your "item.html" modification.
From what I saw, your modifications are pretty clean so I don't think you'd have any problem coming up with a way to reimplement using extensions!
Hope this helps! Good Luck...
- Chris