I'm planning on creating an adventure where the party become trapped in Nessus, the 9th layer of the Nine Hells, and must make their way through the layers of the Nine Hells up to Avernus, the 1st layer, from which they can find a portal and finally escape.
So far, all I have found regarding travelling between layers is this (DMG, pg. 64):
To reach the deepest layer of the Nine Hells, one must descend through all eight of the layers above it, in order. The most expeditious means of doing so is the River Styx, which plunges ever deeper as it flows from one layer to the next. Only the most courageous adventurers can withstand the torment and horror of that journey.
No planar portals connect directly to the lower layers of the Nine Hells, by Asmodeus's orders. As such, the first layer of Avernus is the arrival point for visitors to the plane.
The River Styx, then, seems to connect all of the layers, but I didn't want the party to just sail all the way out, going past all the layers without really experiencing and surviving each one, so I'd rather not use the River Styx if at all possible.
Are there any other ways to travel from one layer of the Nine Hells to another, ideally in descending order (i.e. 9th to 1st)?
This will be for D&D 5e, but information from any previous editions is welcome. If the Hells differs between editions, then assume Forgotten Realms (although information from other settings won't be rejected for not being Forgotten Realms). I also consider the party's level or what I plan on putting in each layer of the Hells for the party to deal with are outside the scope of the question, although I will note that none of them will have access to plane shift or similar.
As an example of the sort of thing I'm looking for, Wikipedia's page on Baator says this about Cania:
Many things lie hidden under the ice of Cania: vast lost cities, frozen armies of creatures. The primary portal to access Nessus is through a gaping hole guarded by 9,999 gelugons. At the bottom of the dizzyingly deep pit is an icy body of water and 1,001 fathoms deep in that is a silvery portal to Malsheem.
However, there are no sources that I can see for this paragraph, so I don't know if any of this is true. But it at least implies that there are portals connecting one layer to another...