I think the definition of what is "anti-democratic" is as hard as the initial 3 questions you pose. If you push second-order ideas, for example by using refugees as indirect fuel for anti-democratic sentiment, is that anti-democratic? The Romanian election propaganda in itself was not anti-democratic, the coordination from a foreign state was. This means that the future of this kind of interference could be a more diffuse approach, or an approach where this is done from within Europe.

Any countermeasure you propose will just lead to moving one level of abstraction, or finding another point of entry.

I do think it's a better idea than mass surveillance, but I believe that the states will see it as harder. It can be that mass surveillance is implemented, and then the states do not know what to do with the data and nothing is achieved.