So many spoilers. Seriously, so many.
I'm skipping the return to the surface entirely, and having the party deal with the demon lords themselves. The ritual will call a specific demon lord to a specific location which has been prepared ahead of time to limit that demon lords power. Examples include disabling/reducing regeneration, reducing save DC's by 5 (Baphomet DC 18 becomes DC 13, Demogorgon DC 23 becomes 18 etc.). Friendly casters could also cast spells to assist the party ahead of time. Being resistant or immune to acid will totally make the Jublex fight a lot less deadly.
To keep things interesting from being summon, fight, repeat. I want to lure Yeenoghu and it's Gnolls into Baphomet and it's minotaur's. They fight it out, and the party mops up the remaining now weakened demon lord. (a friend and I will play out the demon lord fight between games). Juiblex and Zuggtmoy also hate each other, so if the party crashes the wedding by summoning Juiblex there, that could be fun too.
I plan on having Graz'zt in a position of power within Menzoberranzan, having replaced or compelled some of the Drow Matriarchs. The PC's will have to figure out who Graz'zt has replaced or taken control of and shift the balance of power. Shouldn't be too difficult, since I'm reworking Menzonberranzan to be a mixture of horrified Drow and a frat party gone wrong, where hedonism is taking the forefront in a normally reserved society. If Graz'zt gets away, he can always fall back on Gracklstugh where his succubus is still acting as royal consort to the Deepking.
Fraz-Urb-luu is in a gem, so following the bread crumb trail of madness through what is described in the book will be a fun investigation when he doesn't show up when summoned. I might just mess with the party by giving the Fraz Gem as treasure for an unrelated encounter, and then use illusion magic to keep things spicy for a while until they figure it out, and destroy it.
Orcus has reanimated the Elder Brain Cyrog, and taken control of an entire colony of Mind Flayers. I plan on having the PC's have to deal with undead mind flayers. Instead of it being thousands of miles to the East, I plan on making it much closer. Undead Mind Flayers are pushing out and raiding to collect slaves that they can reanimate to grow Orcus' power. Destroy the Elder Brain, free the Mind Flayers. Then the party and the Mind Flayers can tackle Orcus together.
Since the party will have dealt with the Pudding King, and then lured Juiblex away from Blingdenstone, the svirfneblin will be willing to offer some big time support to the party to help defeat Demogorgon directly. If the party has also freed the Drow from Graz'zt, the Drow might offer their support as well. (potential backstab opportunities here). Basically any city that the PC's have garnered enough support from those in power by completing quests in those cities will offer support in the fight. Even if it's just dividing Dempgorgon's attention away from absolutely murdering a PC with focused attacks, that will still help make the fight survivable.