I will note that the Khitani apparently did conquer the Citadel during one of the wars. Considering their distaste for the portals, and it's obvious military use to their enemies, I could see that there used to be a portal there. I fully suspect you can move a portal if you don't care about it still functioning afterward...

So the question might be, if I was a Khitani and took a fortress with a gate in it, where would I put it... Dropping it in the sea sounds good, but it is right next to the Citadel of Yarris, so maybe not... Maybe ship it to their island next door and bury it?

If that were the case, and there is a gate in the Citadel of Saluwe, and there are 4 gates in the First City, that would have provided excellent coverage during the empire.

Not quite. In the 3.5 campaign Portals of Anshar were much more common. So their location was always stated so any player that was collecting them via the Walk the Path of Stars bloodline or by being a cleric of Anshar with the Travel Domain had the opportunity to attune to them. Whether or not they have any relevance to the module itself.
Besides the Spike doesn't come into play in the module The Word of Illiir, it appears in the Hard Point that follows, The Banished. At that point you have left Pecinum and never have the opportunity to explore the city with the Spike in your possession.
The Word of Illiir is designed as a free format investigation module, where the players have the opportunity to gather information about the city and to get to know notable NPCs in the city, including the Ruler of the City at the time. The Commandant of the Scaled Guard.
If a Portal was there it would have been mentioned. You are also on an island it's not like anyone could keep that a secret for so long.

No it makes more sense that if there are 4 portals in the 11 Citadels, that they are all east of the First City. Mind you it is still "if" that 4 exist at all.


Actually a Portal can be moved and still function. In the Soft Point Wrong Turn at Coryan (written by Eric Wiener and M. Sean Molley) a PC can know that 3 Portals had been moved in history and still function. Grand Coryan, Tralia, and Naeraanth.
Perhaps there is a Portal in the Citadel of Larissa. It would make the Jade Box more valuable being able to secretly transport troops from Khitani to a Citadel controlled by them to strike at their enemies.

Yah, I thought we knew it could be moved and keep working, but I knew if that was the case you could move it if you didn't care about keeping it working...

As for the Citadel of Larissa and the Jade Box, I don't understand. The Khitani have (or had) plenty of gates in their own kingdom if they wanted to use them, but since they are associated with Anshar, whom they do not trust, they instead buried or trapped or did something nasty to them, and certainly don't use them. I don't think they would want the Jade Box to use either. So if there is a Gate in the Citadel of Larissa, it question becomes if the Khitani can actually destroy gates, or if they just bury them or something.

It makes me wonder if some less trustworthy Khitani would be willing to sell their gates. After all, they're not using them...

I should point out that there is an example of a gate in former Khitani land: the one outside of Zhuan in HaitiHaina. It wasn't destroyed, it was just abandoned in a forest. I doubt the Khitani would actively destroy a "gift of the gods", but they certainly would let it languish and fall into disrepair and ignore it.