I enjoyed the video, whilst in Kefalonia snorkelling i saw several Octopi , i think they're fascinating creatures. The ones i saw stayed very still, don't think i'd of been so keen if one had shot towards me.
IM so sorry for your loss … it almost sounds like feline leukemia or feline aids... we had 2 litter mates die from it... And the symtoms you describe sound all to familar ... You might contact your vet and ask questions... If you have other cats, this is a highly contagious desease.