Every "experimental" regime was either toppled (Chile) or had to align with the USSR (Cuba) to survive. There was never a real attempt at democratic socialist politics without interference from superpowers.
Idk, but I feel like Olof Palme (PM of Sweden) def got murdered by the USA for his criticism on the Vietnam War. Or by South Africa for his criticism on apartheid.
most governments are allowed to mostly be sovereign
Generally speaking, sovereign governments achieve that sovereignty through military might or the inability of would-be rulers to rule them, not by simply being “allowed” to govern themselves by neighbors.
All Socialism is democratic, including Cuba and the USSR. Trying to reform the system along Socialist lines from within the system like Allende is why he sadly failed and was couped by the US Empire.
The US tried to invade Cuba as well, and tried to kill Castro, several times. That's ultimately why he did align with the USSR - choosing the bully that's slightly more on your side.
This is a non sequitur. It doesn't follow that Allende choosing reform over revolution is what resulted in the US interference. The US has been known to interfere in revolutionary movements as well.