Will it be an ordeal to check the viability of my chimney?

I have an old (1880s) brick house that originally had coal fireplaces. I'd like to retrofit the fireplace in my living room (see pictures) for use with a gas stove or insert. The problem is that the chimney is now occupied by the furnace flue, which runs from the basement up to the 3rd floor roof. I had a chimney guy come out a few years ago and was discouraged from moving forward when he said that he would have to open up the chimney -- presumably through my living room wall -- before he could tell whether the chimney could accommodate another flue.

Why question is, can this be assessed without major surgery on my chimney? And, if it's too difficult to install another flue within my chimney, are there other options available for retrofitting a shallow fireplace to gas that don't look bizarre? I wouldn't mind plunking a gas stove in front of my mantle, but having the vertical vent pipe to one side of the mantle would probably look too off-balance.

Please forgive the old listing pictures! They're not pretty, but they get the point across. Thanks!

I do know that the furnace flue runs through my chimney. What I don't know is if there's room for another flue. So my first question is whether someone will need to tear my wall apart to figure that out. If I can't run another vent through my chimney, then I'll need to figure out alternative ways to vent a gas stove in or around the current mantel that don't look terrible.

All good questions bholler. Just trying to find out who can give me the answers. I don't think that the sweep who last came out went in with a camera, but we were in the middle of a larger renovation project so my memory might be flawed on that.