No one answered, but fortunately I solved my own problem.
For some reason, adding or removing party members in the Gatehouse is completely bugged. At least removing them is; I had to take my party to the tavern to use the chat dialog to release Deekin before proceeding to Aribeth.
To get Aribeth into the party, I had to unsummon my familiar. I used the radial menu to remove the Shield Guardian from the party with the "remove party member" option (I did *not* use unsummon for the Shield Guardian, I don't know if that will make him go away forever). After the Shield Guardian is removed from the party, he is still standing there. They I was then able to get Aribeth join my party. After she joined, I was able to use the control amulet's summon ability to bring the Shield Guardian back into the party.
I also found that if the Shield Guardian is at the top of the henchmen list, he will interfere with certain points at which the henchmen are scripted to start a conversation. It became apparent before going through the gate to go home. Where one of the henchmen is supposed to stop me before going through to start a conversation (which gives the option of using their true names to relieve them of their spiritual burdens), the Shield Guardian would try to start a conversation with me (which only resulted in giving me the menu of his spell slots.) To get the conversations to start properly, I had to use the radial menu to do "remove party member" on the Shield Guardian, then summon him back to the party with the control amulet (thus putting him at the bottom of the henchmen list). I don't know if there were other points in the game where he was at the top of the list and prevented a henchman from talking to me (it seems like I was missing a conversation when the mimic stole my robes). But I guess to be safe, it's best to always move him down to the bottom of the list.
