So Sweden will not join the Euro without a referendum, even though they don’t have an opt out.

Sweden is a member since before the Euro. That’s one of the special treatments pre-Euro member states get. A rejoining UK would be treated like a new member and better come into line.

We could make EMU membership a condition for their EU membership

That’s already a condition. New members cannot opt out of the Euro once the criteria are met.

disproportionate cost of membership

Disproportionate were Thatcher’s UK Rebate and other unfair opt-outs.

Gehirnwäsche ist günstiger und langfristiger als Bestechung.