Its a whale... If this was a... for example 50 ft squid, the mantle would be 40 of those feet and tentacles 10 ft. A 4:1 mantle to tentacle ratio wouldnt be correct. Most cases, arent squid tentacles longer than their mantle?
No, only folklore and anecdotal sightings. The minke in the photograph is not even that far gone and easily recognizable as nothing else but a whale. Globsters are really hard to type morphologically, but DNA or protein analysis consistently show these to be of mammalian origin as well, if we leave the decayed basking sharks aside...
If you do the mechanics, it becomes very hard for something to grow "very" large unless there is an almost unlimited food source and sufficient time at hand. What we know of extant squid and octopus species, suggests that if a huge cephalopod (even larger than M. hamiltoni with a purported maximum weight of 750 kilograms) would exist under the above given constraints, it would most likely have to live in the pelagic zone. Here it would easily be outcompeted by vertebrate predators. Dosidicus is certainly no pushover, albeit no "true" giant either, but its ability to function in a low oxygen environment is likely what gives it its edge in the "almost apex predator" niche. This leads to the second point; if the pelagic zone were to be where we would find truly gigantic cephalopods, breeding population size would dictate frequent sightings to occur.
A third item up for discussion is, that although an aquatic species need not worry about its weight, its mass does not disappear under water. Momentum is a funny thing when you have no internal skeleton and consist mainly of gelatinous- and muscle tissue; beyond a certain size limit an animal is not likely to be able to propel itself without causing trauma to some of its muscle groups or major organs.