Insert in a Prefab fireplace - help needed on options

Last year I had a Napoleon wood stove insert installed. I built out the hearth following all the install instructions. My installer insured me it was OK to put the insert into a Majestic pre-fab fireplace. I had done some research and expressed concern, but he said that the prefab was built to handle direct fires and now the fire is contained, so there is no problem.

I've read all the threads and now know that this should not have been done. So I am at a crossroads with how to fix it.

Given my investment in the Insert, decorative surround and the stonework I did on the interior, I would very much like to re-use the insert and not destroy the stone.

Is it possible to pull out the insert, remove the fireplace from the outside....and then replace the fireplace/chimney with something that is safe to install the insert into? Is there anything made for this scenario? I've seen the cement firebox kits that can be assembled. Am I at a point where I need to rip off everything on the outside and rebuild a mason chimney?

I've done a lot of work on my past three houses and laid a little bit of block. Is this something I should consider taking on myself?

I know that is a lot of questions, but I am looking to develop options quickly.

Thanks for any advice.

Sorry for the delay. I have a 1402. I read all the same info from the manual and that is why I was OK with the install.

The prefab is a Majestic, circa 1980. It does have a 6" stainless liner connected to it.

I guess I am down to 2 concerns at this point.
1.) During the install, the installer cut into a small portion of the inner firebox in the prefab to get the clearance to hook up the stainless liner.
2.) We installed the decorative trim around the outside of the insert. During the install, louvered trims were removed from the prefab fireplace. They appeared to be more decorative than functional, but I didn't know much when I was looking at it.

Is there anything else I can do to increase the safety of this install. The chimney sweep I had here spooked me - he said he wouldn't even touch it since he would then be involved in the chain of custody should something happen.

Thanks for all the insight so far.

6" liner is connected to the insert that is,

He removed a louvered panel on each side of the prefab firebox.

My house was built ~1980 with a Majestic firebox, model ESF - IIA. It has listings for ICBO (RR1581) BOCA(RR75-39) and SBCC(7313)

There was no firebrick in this model

None that I saw. There was a metal plate in the back set off from the back of the box. It wasn't brick, but could that be it?

I'm attaching the front page from the user manual I found for the Majestic. You can see the louvered parts on either side of the firebox. The firebox had a heat exchanger in the back of it. That was left in place.

The sweep seems to have a nearly 100% aversion to inserts in a prefab fireplace. He said unless the specific combination was tested, it shouldn't be done. Also said that the weight of the insert would lead to the firebox buckling at some point. Additionally he said I had some staining on the stainless liner at the top near the cap indicating that I had a chimney fire, the pipe was no longer stainless and would rust. My initial reaction was that he was doing "business development", but he spent 45 minutes here, didn't take a dime and didn't ask for any business. I think he may just be ultra conservative.

Thanks for the compliment on the hearth extension. I spec'ed it out and built it so that the K-factors were all to the install instructions (a lot of 1/2 inch cement board). I really tried to do this thing right, that is why it is frustrating to be told I am putting my family in danger.

I am almost certain that there were no cuts or removal in the back of the box. I did not watch the entire install, but we had multiple conversations about depth and there being just enough to accommodate the insert. Also, I never saw the heat exchanger on the trash pile, so I am pretty certain it is still back there.

As I remember, he made a cut on the inside of the front lip of the firebox to bend the metal up a bit to make room to get the insert in ( I believe the collar for connecting to the flue was not clearing). The installer commented that because it was the "inner" box and the fire will now be contained in a stove (vs. open fire), that it was more than fine.

The comment about the weight didn't calculate for me. I'd say 1/3 of the stove sits on the extended hearth, the next 1/3+ sits over the floor joists resting on the block wall foundation. So there would be some additional weight in the back of the box, but the primary load would rest over the foundation/interior floor joists under the hearth.

I actually believe both of these guys are doing what they honestly feel is correct/safe......just have very different opinions.

Thanks for all the advice/knowledge. This guy has been a sweep for 30 years. He is a bit of a character. Who knows - I don't think he was trying to scare me, but either way he was successful. From our conversation I believe he is 100% against an insert in a prefab, as he condemned it before really even looking at the set up.

The more I recollect/inspect - the only modifications made to the pre fab by the installer were remover of the louver panels, removal of the front threshold so the insert could sit flush and and a couple small cuts in the roof of the inner box so he could bend it back to get the insert to slide in.

A couple more questions:
I am not sure whether the SS liner was insulated. As I recall, he said he was dropping it inside the triple wall chimney that came with the prefab, so it would be fine - but don't remember specifically about insulation. Would I be able to tell if I pulled the chimney cap?

Would it be worth cutting an access panel into my chase and putting a thermometer on a couple spots on the outside of the prefab box to make sure it doesn't get to hot with a stoked fire going? I am thinking that would tell me how hot it is getting on the outside of the fireplace and give me a chance to more full inspect the condition of the fireplace (it looked sound from the front before I put on the lath/scratch coat for the stone).

Thanks again webby3650, begreen and mellow!