There are 7 treasures the PCs can find. They need only 3 to get an audience with Al-Hazred, and 6 to get the extra reward. So the entire sidequest already allows for failures. So I'd propose to simply, let them fail.
I would, however, outright tell the party they don't need every treasure. Tell them they find some extra information in Lowls' notes that they missed before to tell them this. Something like a small note in the margins reading "The Poet was very pleased with so many gifts. He told me most people only ever brought 3 of them, which would be adequate. But with this many, he even gave me a special gift in return!".
This information should make it so your party doesn't just give up after failing one objective.
However, that obviously doesn't work for the Moon-Prison and the Oasis. In addition, you may not want to make escaping an auto-failure for some quests. There are solutions for this.
Time flows differently in the Dreamlands (something that plays a role during the Ghoul-Gug dream per example). So as long as you signal this to your players (again, Lowls notes are the easiest method), it shouldn't be strange that a lot has changed during their previous voyage.
- The viscount is simply holding another ball, and guests still go missing.
- The Ghouls have driven the Gugs out of the catacombs, but the party will have a tough time finding the place (they can not march with the Ghouls this time). The Gugs themselves have risen as undead/skeletal versions during the many years they were left here.
- Sarkath has been destroyed by Bokrug. No two ways about that, unless you want to homebrew an encounter in a rebuilt City of Ib.
- The Ghouls in the ceramic city have been starved and can no longer give any hints or rewards, but the statues are still there, and the Wamps have restored/increased their numbers.
- The Zoogs are still willing to trade, but Nestor has since been sacrificed by the Lurkers-in-Light.
- There is simply no escape from the Bloodwind encounter, it will simply follow the players to the waking world immediately.
- The Night Hag Quaveandra has since found her assassin, and is no longer interested in making a deal like last time. She would be willing to trade her (fake) Heartstone for a powerful soul, though. Either a PC (including familiars or animal companions) or an ally they lure into the dreamlands (Skywin, one of the scholars, etc). If I were feeling particularly mean, I'd have her include Abdul Alhazred to the list, if the players ever mentioned a Mad Poet in her presence. She'd lend the Heartstone in that case to get an audience. Failure to comply (she'd be scrying in on them) will have her appear near the Oasis and complicate the clone-fight.
Of course, this entire course will make simply fighting her the more desirable course of action (unless you have an Evil party). I'd allow the party to find the Assassin's trap and hidden door so they can use it themselves.
As for the Moon-Prison, remember there is no easy way of getting there if you do not have access to Starflight. Teleport doesn't actually work even without Dreamland-logic (you'd need Interplanetary Teleport). There is no ritual to get there, so the players would have to find a way of traveling there from the Caravanserai. Remind them of this fact the moment they start making plans that require them to wake up. In addition, with the volatile timeline, there is no guarantee that the Yellow King will still be alive when they get back.
If it does happen, there is a Moon-beast city not far from the prison, and the Moon-beasts are notorious slavers. It's not a stretch that both sides simply bolstered their numbers by buying combat-slaves. You can simply reset all non-unique encounters, or bring in some other monsters, or even mercenaries with Class-Levels. Maybe Kelvetta Brix or Deanni Khatiri have since been converted into aiding the revolt in exchange for their freedom.
If the PCs try and wake up during the Oasis, I'd let them. They should have gotten all the information they needed (including the optional stat-boost and their memories) before starting the clone-fight, and I'd have Kaklatath contact them as they woke up.
However, I would not give the increased trait-bonus to any PCs that skipped the fight.
If I felt really peeved at my players just skipping an interesting encounter like that (meaning they just run instead of even trying), I'd probably have them wake up with the Dissociated Identity Madness, representing the Oasis-clones trying to take over the PCs bodies. They can get rid of it the normal way, or by travelling to the Dreamlands again and engaging their past selves (this time in an arena of their choosing, since the clones will simply appear in the Dreamlands wherever the party chooses to go).
In either case, I'd tell the players what the clones represent and what they are actually trying to do (probably to the player with the highest Sense Motive, Knowledge Arcana or Heal skill). This should be enough hints that the players can't simply run from this encounter, and that resolving it will help them in the long run.