So what we need is an ICAO IR for PPLs that is acknowledged across the world.
ICAO does "require" mutual acceptance but in practice nearly all countries have washed their hands of it, especially for the IR.
What next your two examples were what I would call "obvious"
The first man was a fool to fly in potentially icing conditions on a VFR flight plan, and the helicopter man was a fool to do an obviously "license illegal" flight.
In brief, on a flight from the UK to Holland, the knowledge that I was flying illegally once in dutch airspace compromised my decision-making & I then concentrated on trying to cover up what I had done when I should have been asking for help
IMHO all that you can do in that situation is declare an emergency, tell them you are instrument capable, and ask for an IAP. I doubt anybody is going to make a fuss. If you just carry on, you are very likely to stick out like a sore thumb because the process of requesting an IFR clearance involves a little bit of terminology which an IMCR pilot may not be familiar with. I know people who got away with this kind of thing but they had the right "lingo", as well as the capability. ATC can usually see from a mile away if somebody is pushing things, and while policing is not their job in most of Europe (I am told German ATC
are a part of the police) somebody might decide to report it.