"I strongly believe that cats mourn when a sibling or a kitten dies, or even another cat in the household who was not related by blood."

Not sure there.

Oedipuss who had been a bit scared of SnotCat was back to “king cat” when he realised the latter had gone. Strutting around and asserting himself…

More generally though, they do seem to know something is amiss and seem appear a little disturbed for a while.

Mind you, cats are funny creatures in another way in that they get used to how things are “supposed to be”. Things like a furniture rearrangement or say with Snotcat either Pip or I not in the living room for the last bit of the night can throw them.