Now for whatever reason I have lost ICS. Don't ask if it is the new beta or what
So without knowing the cause, you blame it on Avast. Not a nice thing to do if you ask me. Pesonally I would use a router to share the connection. If the 'server' system breaks down, none is able to connect, that problem is not there if you use a router.

ICS was nice in the old days when routers where very expensive. Now it is becomming obsolete with the price of decent home routers dropped to about 35 euro (= +/- 35 usd)