Demons are pretty used to explosions. However, I suppose if a prefab concrete thing did explode it would scare the crap out of whoever was carrying it, and that might be good for a laugh, particularly if they guy lost a couple limbs in the process!
If it says it was his most powerful, then it probably was, but he’s not that great at Illusions. But yes, the various “sights” can be used to see through illusions, there are caveats depending on whether the illusion is sensory or mental (sight doesn’t work on mental illusions that well). It’s a question of the level of the illusion and the person trying to see through it, plus they have to be trying to see through it. And then they need to roll a four sided die or something like that.
Things in the Courts that are illusions, are there for the same purpose as a burning log on TV (ok, better quality than that) They aren’t there so much to “hide” something as to make something more pleasant to view. In some cases, they illusions are artwork in intent (e.g. extra cityscape art)
Finally, thank you! Someone realizes that we demons are the Prometheai of Astlan. Teaching those ignorant louts how to do magic!