This is mostly above my skill level. But the ON box is simply transmitting some serial data, as long as your usb adapter, and your program get along it should work...there are NO drivers for this device...therefore you can use whatever you want.
I only have the ON receiver. It does every button exept for the top 3, the volume rocker, and the +/-P rocker.
If you need those buttons, then you need a second receiver. The second one can be anything you wish, it is a standard TV remote at that point.
The entire Qwerty side and all the front buttons (other than the ones mentioned) work just fine with the ON receiver...
I quess the answer to your questions are..
You need the ON IR receiver. (I will take a picture tonight if necessary).
--you need to do some soldering mods on the end of the cable
In addition, for more buttons, you need another IR receiver.
--any standard receiver should work...a USB one for example...if your other ports are being used.
If Girder works using generic plugins, then WinLIRC should work.
I don't know of any substitute for the ON IR receiver(you may have to buy another off ebay, this time WITH the receiver)
It really works great, better than I expected to be honest. (and for about $15 us)