Why didn’t the spirits warn Allan Kardec about the problem of the ETs? It seems reasonable that this may have come up in some form given that many of the communications were not conducted under the purview of the divine realm, thus freeing them from the rule of not leading?

This did not happen for the same reason that the vast majority of humanity is still in the dark about the existence of the ETs, and certainly the level of depravity of their thinking and the dark plans they have in store for you. Humans have been programmed to expect any appearance of an extraterrestrial being to be a blessing. You have been programmed to believe advanced extraterrestrials can only exist because they have overcome the need for warfare, and thus survived the threat of nuclear weapons that has been hanging over humanity as an example of what being warlike leads to. Unfortunately, this is a quite naive assumption and the fact it is deeply ingrained in your beliefs will make humans complacent about anything related to extraterrestrial matters. Most of the spirits communicated with in Kardec's time were spirits within the lower astral plane and, as such, had no more knowledge about esoteric matters than the average citizen and were also in a diminished state, cut off from their normal physical senses, having only their intuitive consciousness to work with that was untutored and functioning at a low level, so communication with higher realms was not even possible for most. So these were beings with limited perspective and awareness. Most of the time, when presumed spirits were communicating coherently with detailed lengthy narratives, these were not spirits at all but extraterrestrial psychics seeking to dupe the listener, keep them complacent, and away from any subjects that could be a threat to the workings of the interlopers themselves. They prevail by keeping you ignorant of their existence and their plans and their capabilities, especially with respect to all the ways they can manipulate you quite readily. To know about this would put you on your guard and they work avidly to discourage and sideline anyone who can see through their defenses and disguises. Any spirit of a truly lofty nature, such as one in the light, was simply not allowed to be leading. As you recognize, it is a hard and fast rule for all in the divine realm, Creator included, to not lead. So until a human being poses the question of whether there are extraterrestrials present, perhaps with evil intentions, and begins to probe with some accuracy about what that might mean, putting two and two together to look at a difficulty in the culture and ask if it is ET related in some way, and so on, the truth will not come forward—it is that simple. Every time through history when people have made observations of alien craft or alien beings, there is mind control manipulation at the ready to convince the witnesses they saw something else that has a trivial explanation, and by instilling the "proper beliefs" will kill the story, so to speak. It is a secret that keeps itself because people are programmed to discount such stories to begin with, so even a lucid accurate observer who witnesses something profound taking place will simply not be believed by others and the story will die with them, having gone no further during their lifetime. Those manipulations have shaped the beliefs of all who transition from the physical into the spirit realm. It is only those back in the light whose eyes are fully awakened and will see the past history of everything once again, know their place in it, and all the dark doings underway throughout the earth plane corrupting and influencing everything that happens, directly and indirectly.