Spain didn't hate Joseph; he had the support of the Church and the Spanish nobility, both of which regarded him (rightly) as an enormous improvement over his predecessor. Spain hated the French army. Napoleon had garrisoned 300,000 of his worst men there, because the "good" men would be needed at the "real" front - so the French army in Spain was a pack of robbers and rapists who couldn't be bothered to (or lacked the capacity to) learn Spanish. Joseph understood this and wrote a couple of letters to his Imperial brother asking him to remove the troops (which were of course ignored). Point being, Emperor Joseph can pull the troops out of Spain and still propose his daughter and/or Lucien as King of Spain, and it's not a ridiculous proposition to the Spanish. (Which doesn't mean it will ultimately succeed, but hey, it's worth a shot.)
I wouldn't count on die-hard British opposition; they've been subsidizing continental powers for 20 years against France and have nothing to show for it. The end of Napoleon is the fig leaf they can use to stop frittering away their money. Besides, it's not as if they've never heard of this Joseph fellow - Britain knows he's a non-aggressive milquetoast, which is the sort of French Emperor they can live with.
The problem, if the Emperor was murdered on his way to sign a peace treaty, will be restraining the Marshals and soldiers on the ground. The one Frenchman with a vested personal interest in the survival of Austria is dead; fair odds they kill every man, woman and child named Habsburg, leave no stone upon stone and salt the earth where Vienna used to be in retaliation. And Austria is defeated again; they won't be able to protect themselves. Although if it goes that way, Britain will still be trying to find allies against France...