The Seventh Coalition was not declared against France, it was declared against Napoleon personally. It was argued that Bonaparte, having escaped from Elba, had 'placed himself outside the pale of civil and social relations' and 'as the disturber of world repose he had exposed himself to public indictment'. As such, the declaration of war was not on France itself, but Napoleon as a person (something that would come back to sting them) and Britain's actions were lawful on account of this joint-declaration by the great powers.

It was agreed amongst the Coalition that Napoleon should be imprisoned in Britain, but the events in the Plymouth Sound - thousands sailed to catch a glimpse of Bonaparte - soon brought that idea to an end. The British Prime Minister, Lord Liverpool, was of course very anxious that the Opposition might capitalise on this new-found popularity and make Bonaparte's imprisonment a political issue - they had attempted to serve a writ of habeas corpus to Napoleon

You're right that the treatment of Napoleon following the Hundred Days and his imprisonment was legally circumspect as the actions of the British Opposition illustrates, which was one of the reasons why Liverpool opted to not bring Napoleon ashore to face the legal questions that would inevitably occur. The first 'easy option' was to hand Napoleon to France and allow Louis XVIII to try him as a rebel, but the chance that this would lead to Napoleon's execution was too high. Instead Liverpool opted to isolate Napoleon so far from Europe that 'all intrigue would be impossible; and, being withdrawn so far from the European world, he would very soon be forgotten'.

Sources:

Adam Zamoyski - Rites of Peace: The Fall of Napoleon & the Congress of Vienna

Harold Nicolson - The Congress of Vienna

John Bew - Castlereagh: Enlightenment, War and Tyranny

Henry Kissinger: A World Restored: Metternich, Castlereagh and the Problems of Peace 1812-1822