Thanks for your hints, CrossRoads.
Opening mind is always good.
On chip debugging was not I was really looking for, currently.
Can do a lot with serial.print, serial.write statements too.
is very correct, but that's somehow on-chip as well !
GitHub - cosaka/Simulino: Arduino simulation project sits on top of PROTEUS
The more I think about my intuitive requirements (behave identical to a real Arduino in terms of code compilation, memory limitations)
my ideas are more about the atmel processor than the arduino built around it.
So perhaps I should go to avr level rather, for my simulation/debugging ideas.
In the avr-gcc toolchain there's also a simulavr.
That's not in the arduino forum scope any more, I think.
Perhaps I spend the minute waiting and play with the " Arduino Simulator " Demo version.