Do you think it can be salvaged? How would you fix it?
The secret lies in the NPCs the adventure and doesn't use.
First off, cut out everything before getting into Hell. We don't need it. And put the players in Elturel when it gets pulled into Hell. Now we're cooking. This is your level 1-5 content, and you have to be attacked immedieatly, and in the distance you need to see Zariel and her armies growing closer to this place (which is swarmed in demons). You don't know what's happening, and all insanity is now on the table, so you got to get out.
So, you get out. Get to the Hag and get your gear somehow. Also at the hag: the last living Hellrider (an NPC in the book) and that holyphant that Zariel used to pal around with.
THE HOLYPHANT DOES NOT TALK. IT DOES NOT TALK. IT DOES NOT TALK. uNLESS your group is down for that, whatever.
Anyway, that Holyphant and Hellrider are being chased by Zariel, cuz they know where her sword is, and she can't let anyone get that sword before her. Zariel doesn't remember why she even wants it, but she found the Holyphant, was going to get its info, and it got saved by the Hellrider somehow. Who knows! Point is, you're traveling with the Hellrider, and Zariel is coming after the Hellrider, so now their problems are your problems.
First, you gotta get somewhere else. Literally anywhere else. ANd the sword can maybe help you. So you hit the road. Combine the Path of Demons and the Path of Devils. Take out all the fetch quest or stuff about having to go do something for someone somewhere else. Its all violent, people trying to trick you so they can take advantage of you with violence, and hidden violence you don't even expect. This is where you bust in the Warlords of Avernus, which are NPCs NEVER USED IN THE WHOLE BOOK REALLY WITH STATS AND WARMACHINE. Combine the Warlords with the violent options of the combined Paths, and have the players run through it. All the way to the end.
And at the end, the Scab. It isn't what they expected. They have someone they can ally with here. I suggest you keep the Dragonborn played by Joe and combine the scab with Tiamat's cave. Maybe the sword is buried in Tiamat's "first corpse" (as she's now smaller and inside her old skull) and by ripping it out of her, she'll be forced to answer one prayer for the players. She wisely tells them they could use her to kill something, or they could use her to help free "their city." Regardless, the players get this, this is their first real moment to rest, and then they dive into the scab.
Do the Scab. Get the sword. Now what? Let'ss change the sword a bit. Any creature attuned with the sword can fly out of Hell. Easy! So now the players could choose to leave, but...if they did...Zariel's plan continues, she consumes an entire city, and the Blood War tips in the favor of devils and things get a lot worse in every world for awhile. Plus, Zariel will chase them to any plane. There's no escape. And they have this potential Dragon Goddess miracle to cash in...
So they turn around AND GO BACK. This is an optional decision though. The players could just leave hell like in Out of the Abyss and never return and that's fine. You now have material for a different homebrew adventure! Yay! But, this is also a natural decision, to go back. I mean, they have the NPC (who could be dead by now, doesn't matter), the sword, the reasons to save the day....the Dragon Goddess miracle...
Run them back. Lots of crazy ass fights. Have their dragon and abishai allies be there. Big bloodshed. End by driving up into Zariel's flying fortress, do a bunch of crazy naughty word, I don't know, explosions, Hell, blood, Doom, etc etc etc. Find Zariel. Beat her, and choose to redeem her or not. If not, she'll be destroyed for good, and Eltruel INSTANTLY returns. Otherwise, if you redeem her, you will have to go to Eltruel and finish what you started.
Let's assume they redeem. Fly with Zariel back to Eltruel. Demons everywhere. The destroyed Zariel army has tipped the war in a just-as-bad direction. Big battle, call in Dragon Goddess to melt the chains or whatever, free the city. Zariel maybe dies in Hell to save the city, or ascends with you all and goes to the Heavens to atone for what she's done, or who knows. Fun adventure! Very Mad Max! Easy to run with the book too, you don't need too much fiddling around to set up the chain.
This adventure works better IMO for levels 5-10. 5 high octane levels where the war machines and other OP stuff you find in Hell lets they should-be-dead characters survive all this insanity. Bring in dark secrets and roleplaying and stuff with the devils as you want. Plan on a lot of set piece combats, a lot of evolving combats, or a lot of shenanigans that replace combats because shenanigans are a pillar of play.
Hope this makes sense and helps!
P.S: The book has AMAZING concept art in it at the end. Use that!!