You're misrepresenting how democracy functions in the PRC and USSR, you're likening their political candidates to what you'd find within a liberal democracy.

I'll talk on Cuba, since it's the socialist democracy I'm most familiar with, but I know its system of representation is fairly similar to how it operates in the PRC and how it worked in the USSR too. People in Cuba do select multiple candidates for their elections, the difference is their politicians don't represent different interests. Rather, the politician is bound by things like public referendum and decisions made by local party members, unions, and regional constituents. Cuban politicians are forbidden by law from proposing policy or advancing a platform. They're forbidden from even doing political campaigns. The most they're allowed to do is post a picture of themselves and their resume. I'm pretty sure the way it works in the PRC is regional chairpeople are nominated and elected in the same way, through selection, referendums, and voting. The only thing they do is bring representation to regional or national assemblies, they aren't independent actors on their own, detached from the communities that elected them.

You're right to say these places have single party domination, but that's because the working class have a single united interest to overthrow capitalism. Why would there need to be two working class parties that compete against one another? Why would there not instead be a single unitary body where issues among the working class can be resolved? Do hospital workers have interests that contradict agricultural workers? No. Do different sectors of capitalist have interests that compete against one another? Absolutely yes, especially between the financial and industrial sectors. This is part of the reason why capitalist nations employ multiple parties, because there are different sectors of competing capitalist factions. Marx talked about this exactly in his essays about the Paris Commune, and any time the petite bourgeoisie are mentioned.

Democracies generally offer a choice? You're saying democracies should allow socialists to take power, or capitalists to take power? Should a socialist country simply have an oppositional liberal/capitalist party just to be fair? Why? Should the USA have a party offering to bring back slavery and return the country to England?

There is exactly one instance, maybe two, of a socialist party getting elected into power and then maintaining it: Czechoslovakia in 1946 and Romania in 1944. Both instances had to have a subsequent coup where the remaining elements of liberal, capitalist, and/or fascist governance had to be purged and both instances required assistance from the USSR. More recent democratic socialist endeavors have shown it's not a terribly viable strategy when under threat of imperialism. Look at Venezuela over the past 20 years and the hell they've had trying to maintain their own national sovereignty. Look at the more recent coup attempt in Bolivia. Look at how the USA killed Salvador Allende in Chile and installed Pinochet.