> If, for example, the State Of California, in the US, were to organize, without constitutional mandate, an "official" referendum calling for succession, Federal Marshall would be all over that in a blink of an eye.

California has, in fact, held initiatives (“referendum” has a particular meaning in California law which is slightly different than it's general usage) on things which violate the federal Constitution. The federal government has done nothing about the votes, though it has a role in challenging and overturning substantive actions taken by the State based on such initiatives after they've passed.

There's no real reason to think a secession initiative would be any different.