It's [megasquid's] weight would crush itself. You said yourself "the size of an elaphant". Why do you think they have huge, THICK bones? (plus you never see a live one on it's side: its organs would collapse, even with it's huge bones)
In fact, elephants can and do occasionally lay on their sides without ill effects. This is one of those common misconceptions that have been printed in literature so often it’s become canon. Elephants have added structural support for their lungs due to a fibrous dense connective tissue which adds protection against pneumothorax. As far as their bones go, yes they are thick, but muscles also add enough support for an animal like this.
As far as the “Future is Wild” issue, having actually corresponded with Dougal Dixon and one of the other scientists who worked on the program, I can say that the explanation had to be watered down substantially to make it a little easier to understand. The “megasquid” was to be supported by means of the interaction between circular and columnar muscles and hydrostatic pressure. The Mega's movement is a little bit wonky for me to accept, and it does look like it would tip over like an SUV on crack. However, such a hydrostatic skeleton is possible, albeit unlikely. I mean, there would have to be one hell of a shift in selective pressures to create terrestrial invertebrate megafauna anyway.
My main issue with the megasquid is that it’s a squid. I mean, given the idea that at 100 million years from now a pulmonate (lunged) octopus has made the transition to land, you would think that maybe somewhere along the line to 200 million years such an octopus would make the transition to a more land-friendly form. If they had done a more complete cladogram or evolutionary path for the squibbons and megasquid, I would have found their explanations more likely. Some intermediate land squid forms might have helped. Then again, CGI is pricey, isn’t it?
As far as parasitic control over a host, look up the word "Rhizocephalan" in any invertebrate zoology text. That is the greatest example of parasite control over a host I can imagine. Oh, and molluscs are usually excellent intermediate hosts for parasites. You might say a "good" parasite usually doesn't kill its host.