Agreed. It's a big unknown and hence a potential safety issue. Finding an alternate location to install a new chimney and the stove would probably be easier than correcting the factors that result in the unknowns and an alternate location would permit the option to use the propane stove, which I doubt meets code either but at least it's been running without burning the house down for the last 25 years!Call it what ever you want it will never pass inspection and is a definite potential safety issue.
Sadly, no one's been able to explain specifically and physically why modern ZC fireplaces with liners are sometimes okay with old AC metal chimneys. (Code doesn't provide reasons.) Of course my interest in the answer stands apart from the other issues specific to the non-conforming chimney and overhang / ceiling or whatever you want to call it.