All of this from the floor color? That can be easily explained by a perpetually-active portal; everything that could affect the coloration of the floor would affect the floor on both sides of the portal. As the portal intercepts anything that would go behind it, the ground in the tunnel behind it would be untouched. Note again that the entry floor color is more muted than the color of the floor by the Order; dustier, maybe, or more worn-down. Hardly proof of Team Evil's tunnel being connected directly to the entrance.
There are hundreds of tunnels, some or all of which might have portals of their own. Even if this is the only tunnel with a trick entrance, the odds of finding that one entrance, even with magical assistance, are pretty low. If the Order has indeed found the way to the Gate, it's probability being a copper piece harlot again.This means that the gate tunnel isn't accidentally accessible to any random adventurer with Detect Magic and Dispel Magic. If the effects are dispelled, the tunnel remains where it is, and the gate too, likely hidden somewhere underground. The teleportation effect is just a convenient service access.
Sensible, if you assume Sereni can't or won't prepare more than one portal. But Kraagor's Tomb is already an example of gross excesses; hundreds of doors, thousands of epic-level monsters, which replenish their numbers if killed, implying stock beyond what we see. Why would the traps be any less excessive?The illusion is just an unconditional one that crosswires the sensory data when observing through the portal.
It also only requires a *single* teleport/illusion combo, instead of one for every tunnel, hundreds or thousands.