Is it possible to configure Internet Sharing to allow the computer to sleep?
I have experimented with using ControlPlane to achieve what you want.  My preliminary result is that it will do what you want.

ControlPlane detects changes to your system called 'evidence sources'.  Changes are evaluated with 'rules' which define a change in 'context'.  Associated with each context are 'actions'.

One of the 'evidence sources' is a sleep/wake event.  So I have configured a rule so that on a sleep event (e.g. you have clicked sleep in the Apple menu), the context 'Sleepy Time' is triggered.  And for this context I have used the action 'ToggleInternetSharing' to off.

After clicking on Sleep, I get an alert saying that Internet Sharing is being turned off and after a little while (seems like forever but is probably only 10 or 20 seconds) the Mac goes to sleep.

And I have a second rule so that on a wake event the context 'Waking Up' is triggered and this has the action to set 'ToggleInternetSharing' on.

My Rules and Actions:




Try it!

This is just scratching the surface of what ControlPlane can do.