From the floor plans, the central chimney stack is a decent depth - the fake brick frontage here will be hiding a nice opening (hopefully!).The fireplace in the second picture is largely a phoney unless the flue behind make a hard dogleg to the left - for obvious reasons the bresummer beam directly above the fire cannot penetrate the chimney stack.
The flues also exit on the ridge line as expected.
Having the spine bearing in the stack is fairly normal, it doesn’t need to penetrate, just get a bearing on the brickwork above the inglenook bressumer, or an oak spreader on the brickwork - this was in our old place, once we restored it…