Actually as a DM there is a single base problem that would thwart your whole idea....
IF.... roy had the ability he wouldnt be able to use it there effectively.
1. the major problem is sand.( if you used that ability there you could spend years going through all the impressions before getting to one that was even partially usable. that sand has blown and moved all over the desert traveling perhaps thousands of miles. so any psy impressions could varitably be from millions of locations. the ability isnt going to be able to map all of that.
any of that sand that has been close enough to a major event in history is going to have some level of an impression on it. It would be garbled at best.

The vision of the event is dreamlike and shadowy. You do not gain special knowledge of those involved in the vision, though you might be able to read large banners or other writing if they are in your language.

i don't think there are going to be any large banners at the entrance to the gate ....

I personally think you are trying to over extend the power of a psionist. haveing played one on table top alot myself your idea just seems like a power gamers attempt to overstep the nature of the ability itself.
Girard my have been highly emotional at the time of the creation of the trap. but also dont forget concentration and being centered are a paramount part of a casters abilities. so the actual creation of the other said illusions to protect the gate where likely done at a time when G was very calm.

I'm not saying it is a complete impossibility. just that the real chance of that ability providing some thing useful would likely take a perfect roll of the dice.