It seems to me that interfering in a foreign election should be understood to be grounds for war.
Yes, and the EU, due to this fragmentation, seems to be a fertile playground for all this unacceptable interference by foreign powers.
Actually, no. The decentralization of power means that it takes a lot more effort to subvert each country individually, rather than propping up a few candidates for the entire region like they do in the US.

That’s true but that fragmentation is also what limits the propagation of fractures. You can see it like sandboxing.

A deal with foreign intelligence is a dead with the devil that comes with a lifetime of subservience. And subservience to foreign powers is a greater evil than yo usual internal corruption. At least the locally corrupt in a democracy have some interest in things going somewhat well in their country. The foreign actors only care about theirs.

No because any attempt at interference would in that case trigger article 5 of NATO.