If you believe anything, believe that these guys do NOT have your best interest in mind. I'm trying to be polite, but this is ridiculous. Let me know who they are, because they certainly have no knowledge of coal burning appliances, or can't read or get advice from others who have decades of experience. I say no to both SS or Thermocrete. No need.
PLEASE make sure the current chimney is what you think it is, and that the clay liner is OK for service. Fly ash is not flammable, however you don't want any CO2 leaking out either.
Look over your chimney, and get another opinion or two before the clowns show up.
Einstein said: The difference between Genius and Idiocy is............................ that Genius has a limit..........
It sounds like theses guys are extending the opposite spectrum....and at your expense.
Thanks for coming to the forum with your questions. We don't have any vested interest in opinions, just experience and facts.
The plywood over the kitchen thimble does make the hair on my neck stand up, but that is going to be a non issue soon.
Safety is no accident.