Polling would be good enough I would think here, do a poll and if you're within 10% of a majority being in favor, just do it and then let them decide on the final agreement. Be public with that stance so that the EU and the population know that the public will have the final say and we (the gov) aren't trying to railroad this
Maybe in a country the size of Iceland it's not as big a deal to run an election but it seems like a lot of work for what is just an advisory vote
This reminds me of a recent vote in the EU parlament where the No vote had to reach a certain value above 50% to carry the day. Even though the vote did end up with a No majority, it passed because the No vote didn't reach the required negativity!
Now that's true democracy.
If Icelanders don't want to join a democracy like that, negotiate until they do! After all, the politicians know what is best for the populace.
As long as they hold a confirmatory referendum before actually joining, it wouldn't matter. Also, if Iceland is a democracy the citizens could simply vote for someone else next time who wouldn't keep trying if that's not what they wanted
It's not a big deal given the major decision regarding even considering losing most of their national soveregnity to one of the most incompetent bureucracies the world has ever seen for hardly any upsides that they can't negotiate individually as a sovereign country.