Here's how I look at the list.
"Unfinished Business" items have taken about a day each, and probably will finish out the same way. The "compat issues" and 40d merge will probably turn out the same, if they compare to past compatibility, mainly because compatibility is always an ongoing issue. The "cat bug" fix will of course be more elaborate than just inventing a bodypart for cats, probably giving creatures with graspers the ability to carry with their mouths in certain circumstances (or something). But for Toady, that's maybe a day's work. Call it 7-10 days for all of those.
As Toady has said, he could work on Underground Stuff forever if he lets himself, which sounds like he won't let himself. Personally, I think ropes and ladders could easily be moved into the other building expansions due for the Expanded Sieges work. But he at least has to polish it off, so it's still indefinite.
Then there's the Hydra Test, hanging bug reports, and the optimistically titled "Any Remaining Tests", which all pertain to putting the game back together and making sure it at least works enough to call releasable. It sounds like Toady has all due diligence in testing items as he goes, but that's also an indefinite time sink.
So, a week-ish plus [indefinite] plus [indefinite]. There's 21 days left in December, including the holidays. I still think he can pull off a releasable (if subtly but hilariously borked) Dwarf Fortress by the 31st.