Based on the discovery so far I think it's safe to assume that the shoddy workmanship continues right up to the chimney top. I would be considering complete removal and replacement with a ZC fireplace or making an alcove for a freestanding stove with a new metal chimney system. My guess is that clearances to the firebox and chimney are incorrect too. Given the poor shape of the brickwork, the fireplace removal could go pretty quickly, working top down from the chimney.
I will let the mason decide who will see what can be done.
Our preference is to avoid blowing this into re-building the whole house thing.
Update: back of the fireplace hearth is gone. BTW, noticed that the first row of cement blocks have different color from the row above. As well, the mortar in the very first row above opening feels different from the row of bricks above, 2nd row.
The 1st row is all loose, mortar is sandy, falls apart when I rub with a finger, while the 2nd row is solid, not cracked nor moving at all, and the mortar is cured very well.
The first row is supported by a flat bar, not steel angle lintel, I don't like it, I can see it's sagging, since the 1st row is completely detached from the 2nd row, the only thing that is holding it in place is that flat bar at the bottom:
BTW, there is some sort of flat bar at the back of the 2nd row as well. I think I will consider replacing the very bottom flat bar with a steel angle lintel, and rebuilding the 1st row.
Which brings me to the fact that the hearth sidewalls of the hearth play some structural role. Except that workmanship suck.
I'm not a mason, but I could have done a better job with my eyes closed - look at those holes there between cement blocks, and fire bricks:
That means I can't just remove them, and will be waiting for the mason to come over and see what the actual plan of attack is.
BTW, before you panic that I might cause a collapse - I'm removing the things that are either loose, and/or are not supporting anything. And so, if we proceed addressing the 1st row, we will build some support for the 2nd row, even though the 1st row isn't even supporting it.