Setting Windows IME input as default input method in XChat is already a possibility:
* The entry box's right-click menu now adds the "Windows IME" input method.
This is useful for inputing various non-English languages. There's a whole
thread on this at viewtopic.php?t=3433
Setting the environment variable GTK_IM_MODULE to "ime" will make this stick.
viewtopic.php?t=3442
#2 is unavoidable unless you delete all other keyboards in your IME, and even then there is still "natural input" and you will have to toggle between hirigana, etc. As this is because of the IME and your operating system nothing can really be done about this by XChat that would help.