It's obvious the authors of FCII read the adventure Fires of Dis -- it's got the Maggot Pit and the red abishai Arraka who guards it. (For some reason, they didn't read the part about the interior of Dispater's tower, but let's not get into that right now.) Yet, despite the fact that FCII is a lot less Planescape than FCI, it actually corrects a few things in Fires of Dis that I never thought made any sense.
Lord Paracs's Passes
In Fires of Dis, it said that Lord Paracs didn't have the legal right to write letters of safe passage through Baator, but everyone used his letters anyway. Not only that, it said the greater baatezu knew this -- and let travelers get away with it! I mean, sure, it could all be some huge fiendish conspiracy to draw Ribcage across the planar border, but the whole point of having letters of safe conduct is so the baatezu would know the who, what, where, when, and why of anyone entering Baator. Otherwise, the plane would be crawling with tanar'ri spies and annoying paladins, and what would that get you? Chaos! (Gasp!) Not only that, what if your party contains Guvners? Would they willingly use letters that they knew weren't legally valid?
(Yes, there's the whole bit about breaking into Dispater's tower, but I'm sure any Guvner worth her salt could rationalize that away. Though, if you're running that adventure with Guvner PCs, you should make them do so to stay in character.)
FCII says you'd need a real letter or a damn good forgery to get past the baatezu authorities, and I say this makes a lot more sense. But this doesn't really mean that Ribcage needs to change much; it just means Paracs doesn't write the letters himself. In Ribcage, you should be able to hook up with people with the right connections to get you a real letter -- or with skilled forgers who can make you one. I hear the Flamen family can help with that -- for a price, of course.
Of course, you still need to go through Paracs's fortress to reach the Cursed Gate. After all, his fortress is as much about keeping baatezu from getting out as it is about keeping mortals from getting in. And, of course, you'd still need to pay him an entry fee -- after all, the fortress is private property. He'd probably even insist on signing your letter of safe passage -- it wouldn't mean anything, of course, but it would make him feel important.
The Entrance to Hell
And now, for the other side of the gate. In Fires of Dis, the other side is guarded by a grand total of one cornugon. Why?! This is the front gate to Hell -- you'd think the baatezu would want to put forth a little effort here. In FCII, it says that the amnizu commonly guard portals into Baator and that they have many organized checkpoints to regulate travelers. The way I see it, the Cursed Gate should open up inside a large fortress filled with amnizu, who would go through the motions of checking your papers, inventorying your possessions, and verifying that you aren't a tanar'ri in disguise. After that, you'd pay the entry fee, they'd stamp your papers, and you'd go on your -- well, not exactly merry -- way.
The thing is, I never thought Planescape really emphasized the lawful nature of Hell. In Mechanus, you'd have to fill out a few minor forms -- or a few thousand -- but you never heard that about Baator. For all the stuff that FCII gets wrong, this is one thing I think it gets right.
I think the authors were actually going for an intentional change of Dis and Dispater's realm.
I think your other points are good, though, and I'd guess Fires of Dis made those errors so as to allow PCs to more easily access Baator and get along with the adventure.
Pants of the North!