It's late (3:20am), but this should be a relatively quick post to write.
With the old tongue-and-groove removed from the chimney cabinet, I was able to start patching all the damaged/rotten boards in the ceiling. This was the first of the two long centre boards:
Staggered joints in the middle.
Some of the short boards were attached with this scrap piece of wood which was installed with screws from above. The screws were installed using a small hand-held ratcheting screw driver.
First two boards. Screwed from above, and nailed at the edge of the wall. A small clamp (seen later) helped keep pieces in place while driving the screws.
3rd and 4th boards.
The last few were nailed to the floor joist.
I used the same system near the bath drain and heating duct corner.
More in a few days!