Wow, where do I start...

smoke shelf damper? just chop it out w/ a cutoff wheel and/or air chisel, sawzall, sledge/BFH... whatever it takes yeah, no going back. but why would you after lining it?

Welcome to the hearth

Welcome to the forums

Does she still have the insert? Is it already lined, or was it a 'slammer"?

I think that the brand of insert will be of some importance here, if memory serves me correctly.

Maybe some pics of the unit and some of the FP, for starters.

I bought a similar house with a similarly installed insert simply dumping into the flue of the chimney. The metal damper was still entirely in place. You can imagine that this setup is difficult to clean and all the creosote easily sets up in the slow moving large throat of the firebox above the insert's flue exit. This is the worst place for creo accumulation since that is where the fire will shoot from the insert to ignite it.

Anyway, I lined the chimney from the cap to the insert with a flexible chimney liner. We cut out the steel damper and made room for the liner to fit. This is how it's done, don't worry about the damper opening being too small. This job hires out for about 1000$ including materials and makes a much safer chimney that can be swept. The insert ran way better going through the liner than the monster chimney flue.

Ultimately I had concerns about the condition of the chimney masonry and tore the whole thing down and replaced it with a freestanding stove and steel pipe. I've been happy with the new setup.