1.) You say mode X, but then you say "each of those modes" You qualify with a certain mode, which you then you have to use in the race. No more, no less. That's also the idea of the parc ferme rules they mention. You're sort of stuck.
2.) You mean equating quali runs as a percentage over the length of a race? That would be unhelpful, then you would get into sort of fanboost type of stuff. I don't read that in the article.
3.) What do you mean by readjust? Not able to change it is how the FIA plan to regulate it under Parc Ferme rules. And they will have some sort of monitoring tool to determine the output of the engines, parameters etc.
You dont qualify only with one engine mode tho, they usually run different engine modes in Q2 than in Q3 for example.
With re-adjusting I mean that if it was only one engine mode they would re-adjust their engine modes before it to still get the maximum out of it, given that Merc has the most powerful and efficient engine I can already tell you what the outcome likely will be.
I'm not sure if your interpretation is correct, because then it's not a quali mode clampdown but a general engine mode clampdown which has way more implications than only qualifying.