You'd have multiple players involved in the same game - that seems to qualify as "multiplayer" to me. This would take resources to develop and bug test which would be resources diverted from the single player game.
When games go multiplayer they sell themselves on co-operative combat. They don't sell themselves on the ability to be random wildlife in another person's game. It's simply beyond reason to imagine a games company would ever invest time and money on such a thing - in the event they ever (please no) did go multiplayer, they would do so on the basis of co-operative play - they would not in a million years try to market Be a spider in someone else's Skyrim! New DLC! Only £20!.
In the bizarre event bethesda ever decided to commit commercial suicide in such a fashion, you'd end up with bored 13 year olds from the other side of the world trolling your game by doing whatever stupid stuff they could dream up.
I didn't say it would not qualify as multiplayer. I said it would not be "multiplayer
AND destroying the genre." In terms of the 'time-invesment' fantasy that you describe -- this can be modded by anyone with access to the full code in less than 2 weeks. True multiplayer games with versus combat or with cooperative gameplay require changes in level design, etc, etc, etc...
There would be no living npc's because player controlled spiders would have killed them all. And the chances of running into the actual player as a spider would be incredibly slim
You would obviously spawn the spider in a region where the player is located, and the players location would be displayed on a minimap. These miniature issues can be dealt with in a variety of ways.