"Most importantly, the police and landowners will do what they want."
This is a large part of the reason for stealth camping itself. While I appreciate the layout of comon law, by the time any of that kicks in I am likely on the loosing side of an argument, whatever it is. So the purpose to stealth camping is to eliminate any question being asked in the first place.
Imagine there is a piece of property that really and truly is mine for the taking. I don't know what it might be, but assume my extensive legal knowledge and careful sleuthing has identified this patch of pure land. If I set up my tent and setttle in, in plain view of the local houses, someone is quite likely to call the police and if they arrive I am probably going to be moving on. Stealth camping just eliminates all the fuss. I'm not sure my position is actually enhanced in any meaningful way by the outward appearance that I have a clear knowledge of the law.
I'm not worried about guys with guns, it always pays to have a love in comon.