A few things...
That would help me as well. Just started replaying this, I'm at the ending Act 2, I can't find the painting in the house to the NW in the Senate District. There was a painting in the Augusus Johnson mansion to the NW, but it did not trigger a convo with Sparky about the painting, didn't think that was it. I went through every house in the Senate District with an opening door (accidentally found teh flawless diamond before triggering the quest, and sold it through the convo).
I don't know if having sold her the diamond already would mean she is no longer interested in the painting, though. (I.e., I don't know if her dialog works if the items are returned out of order.)
I spoke with Desmond a the University and had him open the portal. Followed him through to the Ethereal Plane, but there was no gold dragon there... just Desmond, and his only convo is, "we're here, go ahead and speak with him" - and there's no way to return. Have tried several times but the gold dragon is never there.
When I last went through the module, Desmond never took my party to see Salinder, the dragon. But, I assume that's because we already met him when we respawned after a TPK. If you haven't died and already made your introductions, then I am not sure what's going wrong.
Finally, at the end in the castle... from the conversations after rescuing the Princess, I decided to take back all the gear from teh NPCs when it was clear they were leaving. Even after filling all the bags of holding I had, inventory was full. Wouldn't be a problem, except after I told the Princess I was ready, there was a script to unequip my items before going to the cutscene of the ceremony. Since inventory was full, my best upgraded weapon and shield got dropped, with no opportunity to pick them up before module 3 loaded. Need an opportunity to sell items or buy one more bag of holding (or even a plain backpack/box). Since the only way to get into the castle is through the sewer after a convo with Sparky (who vanishes, IIRC), I suspect if I leave to sell, I won't be allowed back in.
I don't know if there is a convenient way to get a container for your gear before the final fight if you are already in the castle. Your best option may be to use the console to spawn yourself a large box and just assume that you found one laying around when you entered from the sewers. To do the item spawn, open the console by hitting the ` key (usually the key above the Tab key and/or to the left of the numbers on a keyboard) and then entering the following lines at the chat bar.
##DebugMode 1
##dm_spawnitem nw_it_contain001
##DebugMode 0
Before hitting Enter after the second line, make sure the mouse is over some empty ground that the PC can get to.
BTW, the other way to make space for the gear that you en-equip during the cut-scene is to drop some crap out of your inventory. ;-) There is a good chance you don't need all of it and most of your henchmen will have some gear of their own when you re-hire them. Soon after, you could sell their gear to buy replacements for anything they lost. I think that, before long, you generally start finding better gear in Act III anyway.