Part 4 (EDIT: Of Spires of Xin-Shalast) seems to imply, to me, that the best way up from the lower city is the hidden path in area N. However, after they go up and fight the Mountain Roper, there is no Development section to tell me where they can go from there.
Similarly, Part 5 says nothing about where the Hidden Path comes out AFAICS.
What am I missing? Where does the hidden path actually go?
Given the lack of information, where exactly the Mountain Pass comes is DM fiat. My general suggestion is to have it come out anywhere you would specifically like them to go before they reach the Pinnacle of Avarice - in my campaign, it leads directly to a wind-swept ledge with rough stairs ascending the rest of the way to Part 5 Marker T, the Harridan Compound.
My reasoning here is that this hidden path would most likely have been used by lamia moving from Temple Row to the Pinnacle of Avarice (and back) without taking the obvious path through the Rising District. Furthermore, I think Hidden Path -> Harridan Compound -> Malign Ascension -> Pinnacle of Avarice is the most interesting path of approach.
Furthermore, I've been annoyed for quite some time that a party has no opportunity to sell their loot and fully gear up after Xin-Shalast despite receiving a lot of cash loot, so I've added a lost-stop store along the Malign Ascension in the form of Baba Yaga's hut.
Thanks Askar Avari, at least that confirms there was no an official explanation of where it comes out that I missed!
In the end I told the players that they could rejoin the golden road further up or climb the mountain. They took the third option and flew the rest of the way up, bypassing most of the other stuff on the mountain and heading straight into Mockmurian's entrance.