It takes a unanimous vote to admit or expel a country (minus one in the case of expelling), IIUC. The EU couldn't kick Hungary out because Poland wouldn't support it.
It's the same reason a theoretical independent Scotland wouldn't be admitted – there's no way that Spain would want precedent set that a country can split off and then rejoin the EU.
Though I'm cynical enough to doubt if they would follow through – talk is cheap now, as they're not in much danger of Boris Johnson giving Scotland another indyref.
No, I'm pretty sure none of the countries under consideration for being expelled can vote on the decision about the others. I don't have the ref right now as I'm on mobile but this has been discussed before.
You see, it's hard to know in advance all consequences of today's decisions. A person or a political body may have all good intentions, yet with time passing, rules which seems unambiguous and good suddenly are viewed in another light. Many rules can be applied technically correctly, but contrary to the ideas there were when the rules were created.
Sometimes the consequences are seen - there's just no way to avoid them, short of skipping the rulemaking. Sometimes they are not. It seems like legislation, while on the surface similar to software making - only for humans, not computers - in reality works with critically different constraints. That is, computers are relatively easy modeled as systems with clear set of states, while human societies are not.
One may play with the idea to require to attach to every law at the moment of enacting an informal, but complete, as it is feasible, explanation for why the law is created, what its intent and limitations. This will probably won't always work; it may shift problems elsewhere with application - interpretation - of laws though.
Coming back to your question - do you see this scenario you described as visible and deserving attention at the time EU rules were made?
Considering Spain was not a democracy until 1975 and the first EU parliament election was in 1979, I'd say the possibility of an undemocratic state in Europe was not unforeseeable at the time of its formation.
> Considering Spain was not a democracy until 1975
History of Spain reports more than fifty general elections before this date, starting at 1810. Is a fact well documented and impossible to ignore, unless you want to rewrite the history of the country in the last 200 years for some reason. The universal concept of democracy, of course, has changed with time but this was not exclusive from Spain.
People who were on the post-USSR-collapse Fukuyama high of thinking that history was an uninterruptible march towards more liberal democracy and that states would never de-democratize. It seems ridiculous in the cold light of 2020 but it was a pretty common belief in the 90s.
They can still act on it. They just don't have the power to strip Hungary of their voting rights (or theoretically kick them out) without unanimous support.
They can do things like block the Hungarian govt's access to EU funds, bring a case to the European Court of Justice, etc.
It's the same reason a theoretical independent Scotland wouldn't be admitted – there's no way that Spain would want precedent set that a country can split off and then rejoin the EU.