Make sure that the mods' load order is correct (as explained in an answer by DouglasLeeder). If you're unsure on how to do this, just let BOSS take care of the mods' load order. BOSS has a database of mods and their proper load order, and can automatically decide optimal load order of mods, provided they are listed in that database.
Also, see the troubleshooting steps for the 'cannot absorb dragon souls' issue listed at the Unofficial Dragonborn Patch Nexus forums sticky post by Arthmoor, one of the UDGP developers:
Check for: dragonactorscript.pex and/or mqkilldragonscript.pex. Remove them if present. They are from dragon mods that came as loose files.
DSAMG - Dragon Soul Absorb More Glorious, and Diversified Dragons are known to cause this. Those mods need to be updated with a Dragonborn patch that incorporates the fixes from the UDBP.
Skyrim Unbound will cause this as well due to the script being unaware of the changes for Dragonborn.
Others may be a factor as well.
Note too that the offending mod may have the script packaged inside a BSA. That will need to be handled by that mod's author.
If you are using Mod Organizer and are here to report issues with dragon souls, sorry, but you're on your own as we do not support issues caused by incorrectly letting that program modify the BSA load order system the game has. Your post is likely to just be ignored. We don't have time to keep fending off false bug reports caused by people who insist on unpacking their BSA files using the program and thus subverting the entire system the game relies on for proper behavior.
...scripts are baked into saves (right?), so in theory I have to (cringes) restart the game to be absolutely sure...
Yes, scripts are baked into saves. However, there is a way to remove scripts left by uninstalled mods in savefiles. You need to configure Skyrim Script Extender (SKSE) to use its ClearInvalidRegistrations console command. It removes invalid scripts left running by uninstalled mods. This feature was introduced in v1.6.7 of SKSE:
add console command ClearInvalidRegistrations to remove invalid
OnUpdate() registrations
This prevents orphaned OnUpdate() events and
the resulting bloated/broken saves when removing certain mods. When
applied to an already bloated save, it will stop growing further and
instead shrink over time as the game processes all queued events. This
may take hours depending on the amount of bloat.
To execute automatically after each reload, add this to \Data\SKSE\skse.ini:
[General]
ClearInvalidRegistrations=1
Use the skse.ini method to automatically remove invalid scripts left by uninstalled mods.
I read somewhere that No Spinning Death overwrites a script related to dragon death. It might be mucking up things some, but I can't tell.
After configuring SKSE to use the ClearInvalidRegistrations command, try uninstalling the 'No Spinning Death' mod and then see if the issue is fixed. Otherwise, you'll have to disable half your mods and then test again. Rinse and repeat until you find the offending mod.
Best Answer
alright, 4th time around, it worked.
Have to make sure that you don't disintegrate/obliterate any of the three bosses. kill them slowly, until they teleport back to the pedestal. Then Gauldur kills them and reforges the amulet for you.