For what it's worth, I figured out why the fingerboard extension broke off the fingerboard.
There are two reasons actually. The first is the join at the 15th fret was weakened because I had to drill 5 holes to get the steamer into the dovetail. Now that I know how it was glued, I also know not to drill more than 2 holes as I usually would.
The other reason is that the wood around the same fret had swelled quite a bit - due to the steam. So the extension curved downward more than usual. When I reglued it, the angle made it such that it simply wouldn't stay glued - the joint wouldn't flex, so it just snapped.
Anyway, it's not a disaster.
And now I'm going to reglue it to the body and fill the join.
I also did some test fittings and measuring to ensure it lines up. The drafting tape you see is there to use as an alignment.
Ready to glue!
After that, spread the hot hide glue onto the extension and the guitar top.
There's a caul inside the body to protect the big brace that runs under the extension from getting dented.
The radius block serves as the top caul.
A repair like this is an ideal place to use hide glue. It has a high initial tack, and won't let pieces slide like an aliphatic glue would (e.g. Titebond).
As an aside - I mis-wrote something in an earlier post. Hide glue is made from bovine hide, at least in the US. Not equine hide.
Now I'll glue the two chunks you see on the guitar top back into place, and fill any gaps.