Your list of potential causes is a pretty sharp one and one that suggests you know a bit about how engines work. And it remains that all those possibilities could be the root cause of your problem.

If petrol is getting into the sump, it’s going to be past the piston rings, which means you either have a leaky component in the fuel system (that’s dribbling fuel into the engine when it’s not running) or, as you suggest, something like a dud temperature sensor which is creating a too-rich mixture that is not burning completely and contaminating the engine oil in the process.

Having the car scanned might throw up a fault code that coincides with one of these components, and that would be a very good place to start. But don’t rule out the really basic stuff, too, like a worn out engine or a broken piston ring that is allowing fuel to enter the sump. Or perhaps the car is used exclusively for short trips where the constantly cold operating temperature is leading to an equally chronic rich-mixture state.

Meantime, you’re right to not drive the vehicle until this is fixed as the thinned oil now in the sump can’t protect the engine as well as it needs to, leading to accelerated engine wear.