WiFi access without internet sharing?
You want to be able to remotely access a Mac via WiFi, but NOT have that computer be connected to an external network? I'm not sure whether you can do that with the built-in Internet Sharing, because as you noticed it will shut off sharing if there is no connection available to share. However, you ought to be able to solve this with a cheap wireless router. Even with no WAN connection the router should hand out addresses, so you can connect to the Mac by assigned IP.