Thanks for the additional info; it helps a lot. The first item I want to address here is the hole you mention around the furnace flu. Do not insulate and close off this hole; you could be creating a situation for fire. The opening can usually be closed over with a metal plate only, called a fire stop, but I think at the moment I'd leave it be. Instead, I think it's time to call your insurance agent and advise them of what's happened and have an adjustor come look at the home. I'd give them the opportunity too see what's occurred before tearing into things much further.
Then contact a heating contractor, and have them discern whether the flu can use firestops or not. Some flu pipe are double wall, some are not, and this may factor into using a stop. They may advise replacing the flu with double wall pipe if it isn't currently, and then will install firestops, too.
As far as the current ice and water in the attic, my first inclination is to say you might use plastic drape to divert the water away from the ceilings and insulation, but I think I'd do this sparingly, as you don't want to close off these damp areas from airflow and cause more damage. I would also consider running a dehumidifier in the attic on a temporary basis to help draw the moisture out.
In part, it sounds like your problem is the heat rise from this flu, but I'd venture that it also was brought on by lack of attic ventilation near the ridge. Ideally, your attic temperature is supposed to be within 10 degrees, plus or minus, of the outside temperature. Air should flow up into the attic from the eave, and be released at or near the ridge. If theres no escape, heat and humidity will build in the attic, causing problems such as yours.
Post again should you have further question; I hope this helps you.