Use cotton balls wrapped around a doubled and twisted piece of coathanger wire to get inside the holes under the rim when you use the Concrubium.But this is all hygiene theater.

I think your contractor is trying to pull the wool of your eyes. 

Open cell foam will work but only marginally (just a bit) worse than rock wool or fiberglass.

To do this you could cut a piece of rock wool insulation (ether batt or board) to tightly fit into the top of your firebox - I would leave a piece of {bright coloured} string or chain hanging down to remind you the flue is blocked.  

Exterminators were brought in to set off gas canisters during which they had to move out for a few days and in the end they even replaced the expensive wool carpeting.

The only salvation was requiring a mortgage originator to originate non-owner seller financed loans, but those originations seem to be about 350 to 500 bucks, that's not tall cotton!

I used sw Skyline Steel for the walls and felted wool for the trim and floors.

Clean it with mild soap and water and dry with a cotton cloth.

This isn't the first group that came to Indy and thought they were hot sh*t and could pull the wool over the eyes of us Indiana folk and get over on everyone from the municipal government all the way down the line to the out of state investors that line up to get these "deals" that magically get pumped out 20 - 50 a month in a town that barely has that many decent deals total.  

Actually, filling the wall cavity with dense sound deadening insulation (such as rock wool) then face both sides resilient channel (wth 1B-1 or RSIC{?}