(potential spoilers?)

I've finished the trial, as well as the trial of combat- then my character is transported back to the Sunken Flagon.
Screen goes dark, but there's no corresponding cut-scene (I assume there's supposed to be one?) telling my character to go check on Aldanon's Manse. No journal entries either.
The two chests from Lord Nasher and Lorne's belongings are there though.

Even if I go to Blacklake to check out the manse, Cormick isn't around. So I can't progress!

Help, anyone?

Maybe there are still some loose quest ends, which prevent triggering the message that you're wanted at Aldanon's Mansion.

After the trial is over, you find yourself back in the Sunken Flagon with the chest that holds Lorne’s Falchion and a Belt of Hill Giant Strength and because of successfully persuading Lord Nasher to enter a not guilty verdict, another chest that holds the Amulet of Truth.

Now, you can turn to unfinished business.
- West Harbor is available again on the World Map. You can bring the unhappy news about Lorne to Retta Starling for 500xp.
- If you now make a trip to Port Llast, you’ll get ambushed by a rival adventuring party that wants to collect a Luskan bounty on your head.
- In the meantime, you’ll usually get a message that you’re wanted at Aldanon’s Manse which can be accessed through the Blacklake District. After this message, Cormick should be found in front of the building.

The game sometimes gets corrupted while loading areas, thus screwing up savegames. The events of the Trial and the events of the Arena combat were both pretty notorious for that. Therefore it's possible, you have a corrupted savegame file and must fall back to an earlier pre-trial save and do it again. Save often and in different slots.

Thanks for replying! This has been driving me absolutely crazy.
I've tried completing all the outstanding quests (even triggered the Luskan bounty one), to no avail.
And I do keep a lot of saves. Tried reloading one from just before the trial, and still no summons to Aldernon's.
Does Wolf or Duncan act as the quest trigger? I seem to have problems conversing with Duncan (ie: the conversation breaks after he asks me how my travels are faring or somesuch) so I hope its not the latter.

edit:
also, when I exit the tavern post-duel/trial, Shandra is no longer a compulsory member of my party.

I don't know the exact triggers. My guess is, that something went wrong in the savegame, because you seem to have done everything right.

What you can do is figuring out, if there's a flag which triggers the hostage situation. But this is a complete shot in the dark.
Load a savegame, after the trial in the Sunken Flagon, save again and quit the game.
In the \saves\ folder, you'll find all your savegames. Choose the one you just saved and look for the globals.xml file.
Make a backup and open it with any text editor.
Unfortunately, I don't have a savegame at this stage anymore, but one at end-game. Hence all flags in my global.xml are set to the state after finishing the game. Anyway, two entries might relate to the Sage's Manse quest:

<Name>
__conv20_cs_hostage_standoff0
</Name>
<Value>
1
</Value>

and

<Name>
__conv20_aldasst_hostage0
</Name>
<Value>
1
</Value>

You can try the values 1 and 0 and see what it does in-game. If you're very lucky, the hostage situation arises.
But still, if the save is corrupt, you might experience problems further on.