Oh. I am not very familiar with Homestruck's canon. I tried to do a bit of investigating over the topic, but I do not understand half of the information found. Homestruck is pretty difficult canon to follow, it would seem.

Either way, the Homestruck trolls are aliens from another world, right? If it is part of Kanaya's biology to notice the supernatural or be somewhat related to it, then I believe she might be able to see it. I will go with the basis that she isn't human nor from the same planet, so she should be able to see it. The only creatures unable to see the sign (even if they are part of the supernatural) are normal human beings. Other demons are a split because some have notice it while others do not because they have a superior complex notion against humans.

If Kanaya does not have any bias over humans being nothing special, then she will be unable to see it. However, if she does believe humans can be special or has a neutral view of them, then she will be able to see it. The Kamisama Hajimemashite canon have their own odd rules, but I believe they are used to separate the human and demon/god world.

Yes. I believe I will go with the above notion that non-human or non-Earth characters can easily notice the deity seal as long as they do not have a negative opinion of humans.

Does that idea or explanation help?