I got my IA-1 off Ubid aswell, I've had mine about a week and played a litle but had to box it up for a few weeks because I'm moving. Some answers to your questions from what I've read and what litle I've done.
1. WinCE will indead run on an IA-1 but you will need to make a custom version using Platform builder to do so, and WinCE does support USB eathernet addapters.
2. Yes the OS is a custom version of CE, but no there dosn't seem to be a way to just get around what they have done to it.
3. No one apparently has been able to get Jailbait to work with the USB adapters on the IPaq and they can't get the sound to work either, along with alot of other problems.
4. YES very much so, there is a good deal of info across this board but very litle centralization (there is other info in places other than the MSN Companion section).