Hi, I'll try to answer your questions.
1. disks failing in NASLite will act the same as disks failing in Windows. If you have two disks and one fails, the other one will still have data so you can copy it to a new drive. If you remove Disk-1 and reboot NASLite it will just say NOT PRESENT or something like that in the admin screen. DISK-2 will still be accessable.
2. yes & yes. This is more of a problem due to your hardware and how cleanly it handles abrupt power removal instead of a NASLite issue. But NASLite will try to fix any corruption the next time it starts. This can take several hours for large disks. I have my NASLite system on a UPS which gives me time to shut down if the power goes out and protects me from short power glitches. Won't help you at the office if power goes out for long periods during the night though
3. should work, but transfers may be slower than with a wired setup and you may have issues with your XP machines "forgetting" about NASLite if there is interference and you drop the wireless connection to the NASLite box.
4. No, only by the computers on the same network.
Hope this helps.