It's usually better to pay in the local currency than Euro. The currency still has to be exchanged somewhere, and the banks usually have better rates than the terminal operators.
The lack of direct documentation/instructions or link in the terminals to official rules strikes me as horrible ux. You basically just have to be in the know or know who to ask?
In Germany, there was a sliver of time where stores essentially taped a piece of paper to terminals with instructions and big red arrows to select a specific scheme because that would benefit the store (and not cost the consumer extra). It didn't really stick, however. I suspect because it was two extra button presses and the consumer wouldn't notice either way.