The drain seen in Figure 1 appears to me to have been modified from the original. It looks like it has a section of plastic pipe inserted. You can see a distinct change in material between the hull gelcoat and the drain tube. Or perhaps what appears is just some old sealant.
Typically a hull drain would not be drilled through the transom area with embedded wood reinforcement.
Q5: are you certain that the hull material surrounding this drain is wood?
Also, replacement of drain tubes is a topic in the Frequently Asked Questions answers. See the answer at:
Q12: How Are brass drain tubes replaced?https://continuouswave.com/whaler/reference/FAQ/#Q12If the drain is actually drilled through wood, and the wood is rotting, you should use a rot-stopping treatment like GET-ROT to stabilize the wood. Then you could enlarge the hole slightly, and coat the wood interior with epoxy. Then re-drill the hole to the proper size to fit the replacement brass drain tube. Don't seal the entire tube in place with sealant. Don't use a super-adhesive sealant like 3M4200 or 3M5200. Just use a marine caulk at the ends of the flared tube if you don't get a good seal with the rubber O-rings.
"GIT" ROT
https://defender.com/en_us/boatlife-git-rot-wood-repair