Ukraine's plans to reconquer Crimea militarily may be impossible or unwise.
The places that Russaian troops were stationed since 2014 include, as
a fact, parts of the Donetsk and Luhansk regions, but Putin always
denied that this was the case. He should not be invited to
acknowledge it belatedly now and reap any benefit.
To negotiate the fate of Crimea is more sensible. An honest election
would have been a legitimate approach in 2014, but Putin drove out or
exiled those who supported Ukraine, then (according to this article)
lost anyway and lied about the result.
Putin's elections in Russia are blatantly dishonest.
The population of Crimea today is different from the population in
2014. If an election is to be held now about which country Crimea
should belong to, which people should vote in it? To give the
decision to the current population would mean that Putin (or anyone
else) can conquer territory and legitimize that with an unfair
election. To give the decision to the 2014 population might be asking
for further violence.